Gallium nitride self-supported substrate, light-emitting device and manufacturing method therefor

ABSTRACT

Provided is a self-supporting gallium nitride substrate useful as an alternative material for a gallium nitride single crystal substrate, which is inexpensive and also suitable for having a large area. This substrate is composed of a plate composed of gallium nitride-based single crystal grains, wherein the plate has a single crystal structure in the approximately normal direction. This substrate can be manufactured by a method comprising providing an oriented polycrystalline sintered body; forming a seed crystal layer composed of gallium nitride on the sintered body so that the seed crystal layer has crystal orientation mostly in conformity with the crystal orientation of the sintered body; forming a layer with a thickness of 20 μm or greater composed of gallium nitride-based crystals on the seed crystal layer so that the layer has crystal orientation mostly in conformity with crystal orientation of the seed crystal layer; and removing the sintered body.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/499,688, filed Sep. 29, 2014, the entirety of which is incorporatedherein by reference, which in turn is a continuation-in-part ofInternational Application No. PCT/JP2014/064388, filed May 30, 2014,which designated the United States.

FIELD OF THE INVENTION

The present invention relates to a self-supporting gallium nitridesubstrate, a light emitting device, and manufacturing methods therefor.

BACKGROUND OF THE INVENTION

As light emitting devices such as light emitting diodes (LEDs) in whicha single crystal substrate is used, light emitting devices in whichvarious gallium nitride (GaN) layers are formed on sapphire (α-aluminasingle crystal) are known. For example, those having a structure formedby stacking on a sapphire substrate an n-type GaN layer, a multiplequantum well (MQW) layer in which a quantum well layer composed of anInGaN layer and a barrier layer composed of a GaN layer are alternatelystacked, and a p-type GaN layer in this order are in mass production.Moreover, a multi-layer substrate suitable for such use is alsoproposed. For example, Patent Document 1 (JP2012-184144A) proposes agallium nitride crystal multi-layer substrate including a sapphire basesubstrate and a gallium nitride crystal layer formed by crystal growthon the substrate.

When a GaN layer is formed on a sapphire substrate, dislocation islikely to occur because the lattice constant and the coefficient ofthermal expansion of the GaN layer do not match with those of sapphire,which is a foreign substrate. Moreover, since sapphire is an insulatingmaterial, it is not possible to form an electrode on its surface, and,therefore, it is not possible to configure a light emitting devicehaving a vertical structure that includes electrodes on the front andback of the device. Accordingly, LEDs in which various gallium nitride(GaN) layers are formed on a GaN single crystal have been attractingattention. Since a GaN single crystal substrate is made of the same typeof material as a GaN layer, the lattice constants and the coefficientsof thermal expansion are likely to match, and higher performance can beexpected than the case where a sapphire substrate is used. For example,Patent Document 2 (JP2010-132556A) discloses a self-supporting n-typegallium nitride single crystal substrate having a thickness of 200 μm orgreater.

CITATION LIST Patent Document Patent Document 1: JP2012-184144A PatentDocument 2: JP2010-132556A SUMMARY OF THE INVENTION

However, single crystal substrates in general have small areas and areexpensive. In particular, while there are demands for reduction ofproduction costs of LEDs in which large-area substrates are used, it isnot easy to mass-produce large-area single crystal substrates, and doingso results in even higher production costs. Accordingly, an inexpensivematerial that can be an alternative material for single crystalsubstrates of gallium nitride or the like is desired.

The inventors have currently found that a self-supporting galliumnitride substrate that is inexpensive and also suitable for having alarge area can be produced as an alternative material for a galliumnitride single crystal substrate.

Therefore, an object of the present invention is to provide aself-supporting gallium nitride substrate useful as an alternativematerial for a gallium nitride single crystal substrate, which isinexpensive and also suitable for having a large area.

According to an aspect of the present invention, there is provided aself-supporting gallium nitride substrate composed of a plate composedof a plurality of gallium nitride-based single crystal grains, whereinthe plate has a single crystal structure in an approximately normaldirection.

According to another aspect of the present invention, there is provideda light emitting device comprising:

the self-supporting gallium nitride substrate according to the presentinvention; and

a light emitting functional layer formed on the substrate, wherein thelight emitting functional layer has at least one layer composed of aplurality of semiconductor single crystal grains, wherein the at leastone layer has a single crystal structure in an approximately normaldirection.

According to yet another aspect of the present invention, there isprovided a method for manufacturing a self-supporting gallium nitridesubstrate, comprising the steps of:

providing an oriented polycrystalline sintered body;

forming a seed crystal layer composed of gallium nitride on the orientedpolycrystalline sintered body so that the seed crystal layer has crystalorientation mostly in conformity with crystal orientation of theoriented polycrystalline sintered body;

forming a layer with a thickness of 20 μm or greater composed of galliumnitride-based crystals on the seed crystal layer so that the layer hascrystal orientation mostly in conformity with crystal orientation of theseed crystal layer; and

removing the oriented polycrystalline sintered body to obtain theself-supporting gallium nitride substrate.

According to yet another aspect of the present invention, there isprovided a method for manufacturing a light emitting device, comprisingthe steps of:

providing the self-supporting gallium nitride substrate according to thepresent invention; and

forming on the self-supporting gallium nitride substrate at least onelayer composed of a plurality of semiconductor single crystal grains,wherein the at least one layer has a single crystal structure in anapproximately normal direction, so that the at least one layer hascrystal orientation mostly in conformity with crystal orientation of thegallium nitride substrate, thereby providing a light emitting functionallayer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional diagram showing one example of avertical light emitting device produced using the self-supportinggallium nitride substrate of the present invention.

FIG. 2 is an inverse pole figure map of the cross section of galliumnitride crystals measured in Example 4.

FIG. 3 is an inverse pole figure map of the plate surface (top surface)of gallium nitride crystals measured in Example 4.

FIG. 4 is a crystal grain map near the interface between gallium nitridecrystals and an oriented alumina substrate measured in Example 4.

FIG. 5 shows conceptual diagrams of the growth behavior of galliumnitride crystals assessed in Examples 4 and 5.

FIG. 6 is an inverse pole figure map of the cross section of galliumnitride crystals measured in Example 5.

DETAILED DESCRIPTION OF THE INVENTION Self-Supporting Gallium NitrideSubstrate

The gallium nitride substrate of the present invention can take the formof a self-supporting substrate. In the present invention, the“self-supporting substrate” means a substrate that does not becomedeformed or damaged by its own weight when handled and that can behandled as solid matter. The self-supporting gallium nitride substrateof the present invention is usable as a substrate for varioussemiconductor devices such as light emitting devices, and, in addition,it is usable as a component or a layer other than a substrate, such asan electrode (which may be a p-type electrode or an n-type electrode), ap-type layer, or an n-type layer. In the following description,advantages of the present invention may be described by way of a lightemitting device as an example, which is one of the principalapplications, but similar or analogous advantages are also applicable toother semiconductor devices as long as such advantages are nottechnically contradictory.

The self-supporting gallium nitride substrate of the present inventionis composed of a plate composed of a plurality of gallium nitride-basedsingle crystal grains, wherein the plate has a single crystal structurein the approximately normal direction. That is, the self-supportinggallium nitride substrate is composed of a plurality of semiconductorsingle crystal grains connected two-dimensionally in the direction in ahorizontal plane, and, therefore, has a single crystal structure in theapproximately normal direction. Therefore, although the self-supportinggallium nitride substrate is not a single crystal as a whole, theself-supporting gallium nitride substrate has a single crystal structurein terms of local domains, and can therefore have sufficiently highcrystallinity for ensuring device properties such as light emittingfunction. Yet, the self-supporting gallium nitride substrate of thepresent invention is not a single crystal substrate. As described above,single crystal substrates in general have small areas and are expensive.In particular, while there are demands in recent years for reduction ofproduction costs of LEDs in which large-area substrates are used, it isnot easy to mass-produce large-area single crystal substrates, and doingso results in even higher production costs. These drawbacks areaddressed by the self-supporting gallium nitride substrate of thepresent invention. That is, the present invention can provide aself-supporting gallium nitride substrate useful as an alternativematerial for a gallium nitride single crystal substrate, which isinexpensive and also suitable for having a large area. Moreover, byusing gallium nitride having electroconductivity due to introduction ofa p-type or n-type dopant as a substrate, it is possible to achieve alight emitting device having a vertical structure, and it is therebypossible to enhance luminance. In addition, a large-area surface lightemitting device for use in surface emitting lightings or the like can beachieved at low cost. In particular, when a vertical LED structure isproduced using the self-supporting gallium nitride substrate of thepresent invention, because the plurality of gallium nitride-based singlecrystal grains constituting the self-supporting substrate have a singlecrystal structure in the approximately normal direction, highlyresistive grain boundaries do not exist in electrical current paths, andas a result, preferable luminous efficiency is expected. In this regard,in the case of an oriented polycrystalline substrate in which grainboundaries exist also in the normal direction, highly resistive grainboundaries exist in electrical current paths even when a verticalstructure is formed, and thus there is a possibility of impairedluminous efficiency. From these viewpoints, the self-supporting galliumnitride substrate of the present invention can be preferably used alsofor a vertical LED structure.

Preferably, the plurality of gallium nitride-based single crystal grainsconstituting the self-supporting substrate have crystal orientation thatis mostly aligned in the approximately normal direction. The “crystalorientation that is mostly aligned in the approximately normaldirection” is not necessarily limited to crystal orientation that iscompletely aligned in the normal direction, and means that it may becrystal orientation that is, to some extent, in alignment with thenormal or a direction similar thereto as long as desired deviceproperties of devices such as light emitting devices including theself-supporting substrate can be ensured. Using an expression derivedfrom the production method, it can also be said that the galliumnitride-based single crystal grains have a structure in which grains aregrown mostly in conformity with the crystal orientation of an orientedpolycrystalline sintered body used as a base substrate in producing theself-supporting gallium nitride substrate. The “structure in whichgrains are grown mostly in conformity with the crystal orientation of anoriented polycrystalline sintered body” means a structure resulting fromcrystal growth influenced by the crystal orientation of the orientedpolycrystalline sintered body, is not necessarily limited to a structurein which grains are grown completely in conformity with the crystalorientation of the oriented polycrystalline sintered body, and may be astructure in which grains are grown, to some extent, in conformity withthe crystal orientation of the oriented polycrystalline sintered body aslong as desired device properties of devices such as light emittingdevices in which the self-supporting substrate is used can be ensured.That is, this structure also includes a structure in which grains aregrown in crystal orientation different from that of the orientedpolycrystalline sintered body. In this sense, the expression “structurein which grains are grown mostly in conformity with crystal orientation”can be paraphrased as “structure in which grains are grown in a mannermostly derived from crystal orientation”, and this paraphrasing and theabove meaning similarly apply to similar expressions in thisspecification. Therefore, such crystal growth is preferably epitaxialgrowth, but it is not limited thereto, and may take a variety of similarcrystal growth forms. In any case, with crystals grown in this way, theself-supporting gallium nitride substrate can have a structure, thecrystal orientation of which is mostly aligned with respect to theapproximately normal direction.

Therefore, the self-supporting gallium nitride substrate is observed asa single crystal when viewed in the normal direction, and it is alsopossible to recognize it as an aggregate of gallium nitride-based singlecrystal grains having a columnar structure in which grain boundary areobserved in a view of the cross section in the horizontal planedirection. Here, the “columnar structure” does not mean only a typicalvertically long columnar shape, and is defined as having a meaningencompassing various shapes such as a horizontally long shape, atrapezoidal shape, and an inverted trapezoidal shape. As describedabove, the self-supporting gallium nitride substrate may have astructure with crystal orientation that is, to some extent, in alignmentwith the normal or a direction similar thereto, and does not necessarilyneed to have a columnar structure in a strict sense. As described above,the growth of gallium nitride single crystal grains due to the influenceof the crystal orientation of an oriented polycrystalline sintered bodyused for production of a self-supporting gallium nitride substrate isconsidered to be the cause of the columnar structure. Therefore, theaverage grain diameter at the cross section (hereinafter referred to asa cross-sectional average diameter) of gallium nitride single crystalgrains that can also be called columnar structures is considered todepend on not only the conditions of film formation but also the averagegrain diameter at the plate surface of the oriented polycrystallinesintered body. In the case where the self-supporting gallium nitridesubstrate is used as a part of a light emitting functional layer of alight emitting device, the presence of grain boundaries impairs lighttransmittance in the cross-sectional direction and causes light to bescattered or reflected. Therefore, in the case of a light emittingdevice having a structure in which light is extracted in the normaldirection, a luminance increasing effect due to scattered light fromgrain boundaries is also expected.

As described above, in the case where a vertical LED structure is formedusing the self-supporting gallium nitride substrate of the presentinvention, it is preferable that the top surface of the self-supportingsubstrate on which a light emitting functional layer will be formed andthe bottom surface of the self-supporting substrate on which anelectrode will be formed connect without intervention of a grainboundary. That is, it is preferable that the gallium nitride-basedsingle crystal grains exposed at the top surface of the self-supportinggallium nitride substrate connect to the bottom surface of theself-supporting gallium nitride substrate without intervention of agrain boundary. The presence of a grain boundary causes resistance whenelectricity is applied, and therefore becomes a factor that deterdeteriorates luminous efficiency.

Meanwhile, when gallium nitride crystals are grown using epitaxialgrowth via a vapor phase or a liquid phase, growth occurs not only inthe normal direction but also in the horizontal direction, depending onthe conditions of film formation. At this time, if the quality of grainsthat serve as a starting point of growth or of seed crystals producedthereon varies, the growth rates of respective gallium nitride crystalsdiffer, and, as conceptually shown in, for example, FIG. 5, fast-growinggrains may grow to cover slow-growing grains. In the case of such agrowth behavior, grains on the top surface side of the substrate arelikely to have a larger diameter than those on the bottom surface sideof the substrate. In this case, growth of slow-growing crystalsterminates halfway, and a grain boundary can be observed also in thenormal direction when a certain cross section is observed. However, thegrains exposed at the top surface of the substrate connect to the bottomsurface of the substrate without intervention of a grain boundary, andthere is not a resistive phase against application of an electriccurrent. In other words, after gallium nitride crystals are formed intoa film, the grains exposed on the top surface side of the substrate (theside opposite to the side that was in contact with the base-substrateoriented polycrystalline sintered body during production) arepredominantly grains that connect to the bottom surface withoutintervention of a grain boundary, and therefore it is preferable toproduce a light emitting functional layer on the top surface side of thesubstrate from the viewpoint of increasing the luminous efficiency of anLED having a vertical structure. On the other hand, on the bottomsurface side of the substrate (the side that was in contact with thebase-substrate oriented polycrystalline sintered body duringproduction), there are also grains that do not connect to the topsurface of the substrate (see, for example, FIG. 5), and thus there is apossibility of impaired luminous efficiency if a light emittingfunctional layer is produced on the bottom surface side of thesubstrate. Moreover, as described above, in the case of such a growthbehavior, grains develop to have a large diameter as they grow, andtherefore, the top surface of the self-supporting gallium nitridesubstrate can be paraphrased as the side on which the grain diameter ofgallium nitride crystals is larger, and the bottom surface thereof canbe paraphrased as the side on which the grain diameter is smaller. Thatis, in the self-supporting gallium nitride substrate, it is preferableto produce a light emitting functional layer on the side where the graindiameter of gallium nitride crystals is larger (the top surface side ofthe substrate) from the viewpoint of increasing the luminous efficiencyof an LED having a vertical structure. When an oriented polycrystallinealumina sintered body that is oriented along the c-plane or the like isused for a base substrate, the top surface side of the self-supportinggallium nitride substrate (the side opposite to the side that was incontact with the base-substrate oriented polycrystalline aluminasintered body during production) becomes the gallium surface, and thebottom surface side of the self-supporting gallium nitride substrate(the side that was in contact with the base-substrate orientedpolycrystalline alumina sintered body during production) becomes thenitrogen surface. That is, at the gallium surface of the self-supportinggallium nitride substrate, grains connecting to the bottom surfacewithout intervention of a grain boundary are predominant. Therefore, itis preferable to produce a light emitting functional layer on thegallium surface side (the top surface side of the substrate) from theviewpoint of increasing the luminous efficiency of an LED having avertical structure.

Therefore, in the case where grains on the top surface side of thesubstrate exhibit such a growth behavior that their grain diameter islarger than that of grains on the bottom surface side of the substrate,or that is to say, in the case where the cross-sectional averagediameter of gallium nitride-based single crystal grains exposed at thetop surface of the substrate is larger than the cross-sectional averagediameter of gallium nitride-based single crystal grains exposed at: thebottom surface of the substrate, luminous efficiency is increased, andtherefore such diameters are preferable (this can be paraphrased that itis preferable that the number of gallium nitride-based single crystalgrains exposed at the top surface of the substrate is smaller than thenumber of gallium nitride-based single crystal grains exposed at thebottom surface of the substrate). Specifically, the ratio D_(T)/D_(B),which is the ratio of the cross-sectional average diameter at theoutermost surface of gallium nitride-based single crystal grains exposedat the top surface of the self-supporting gallium nitride substrate(hereinafter referred to as the cross-sectional average diameter D_(T)at the top surface of the substrate) to the cross-sectional averagediameter at the outermost surface of gallium nitride-based singlecrystal grains exposed at the bottom surface of the self-supportinggallium nitride substrate (hereinafter referred to as thecross-sectional average diameter D_(B) at the bottom surface of thesubstrate), is preferably greater than 1.0, more preferably 1.5 orgreater, even more preferably 2.0 or greater, particularly preferably3.0 or greater, and most preferably 5.0 or greater. However, anexcessively high D_(T)/D_(B) ratio may in turn result in impairedluminous efficiency, and therefore a ratio of 20 or less is preferable,and 10 or less is more preferable. Although the cause of change inluminous efficiency is not clear, it is considered that when the ratioD_(T)/D_(B) is high, the area of grain boundaries that do not contributeto light emission is reduced due to the increased grain diameter, orcrystal defects are reduced due to the increased grain diameter.Although the cause of reduction in crystal defect is not clear either,it is also considered that defective grains grow slowly, and grains withless defects grow fast. On the other hand, when the ratio D_(T)/D_(B) isexcessively high, the cross-sectional diameter of grains that connectbetween the substrate top surface and the substrate bottom surface(i.e., grains exposed at the top surface side of the substrate) is smallnear the bottom surface side of the substrate. As a result, sufficientelectric current paths are not obtained, which can be considered as acause of reduction in luminous efficiency, but the details thereof arenot clear.

Crystallinity at the interface between columnar structures constitutingthe self-supporting gallium nitride substrate is low, and therefore whenthe self-supporting gallium nitride substrate is used as a lightemitting functional layer of a light emitting device, there is apossibility that the luminous efficiency deteriorates, the emissionwavelength changes, and the emission wavelength broadens. Thus, a largercross-sectional average diameter of the columnar structures ispreferable. Preferably, the cross-sectional average diameter ofsemiconductor single crystal grains at the outermost surface of theself-supporting gallium nitride substrate is 0.3 μm or greater, morepreferably 3 μm or greater, even more preferably 20 μm or greater,particularly preferably 50 μm or greater, and most preferably 70 μm orgreater. Although the upper limit of the cross-sectional averagediameter of the semiconductor single crystal grains at the outermostsurface of the self-supporting gallium nitride substrate is notparticularly limited, it is realistically 1000 μm or less, morerealistically 500 μm or less, and even more realistically 200 μm orless. In order to produce semiconductor single crystal grains havingsuch a cross-sectional average diameter, it is desirable that thesintered grain diameter at the plate surface of grains that constitutethe oriented polycrystalline sintered body used for producing theself-supporting gallium nitride substrate is 0.3 μm to 1000 μm, moredesirably 3 μm to 1000 μm, even more desirably 10 μm to 200 μm, andparticularly desirably 14 μm to 200 μm. Alternatively, with a view tomaking the cross-sectional average diameter of semiconductor singlecrystal grains at the outermost surface of the self-supporting galliumnitride substrate larger than the cross-sectional average diameter atthe bottom surface of the self-supporting substrate, it is desirablethat the sintered grain diameter at the plate surface of grainsconstituting the oriented polycrystalline sintered body is 10 μm to 100μm and more desirably 14 μm to 70 μm.

The gallium nitride-based single crystal grains constituting theself-supporting gallium nitride substrate may be free from a dopant.Here, the phrase “free from a dopant” means that an element that isadded to impart a certain function or property is not contained, but,needless to say, inevitable impurities are allowed. Alternatively, thegallium nitride-based single crystal grains constituting theself-supporting gallium nitride substrate may be doped with an n-typedopant or a p-type dopant, and, in this case, the self-supportinggallium nitride substrate can be used as a component or a layer otherthan a substrate, such as a p-type electrode, an n-type electrode, ap-type layer, or an n-type layer. Preferable examples of p-type dopantsinclude one or more selected from the group consisting of beryllium(Be), magnesium (Mg), calcium (Ca), strontium (Sr), zinc (Zn), andcadmium (Cd). Preferable examples of n-type dopants include one or moreselected from the group consisting of silicon (Si), germanium (Ge), tin(Sn), and oxygen (O).

The gallium nitride-based single crystal grains constituting theself-supporting gallium nitride substrate may be formed into mixedcrystals for band gap control. Preferably, the gallium nitride singlecrystal grains may be composed of gallium nitride formed into mixedcrystals with crystals of one or more selected from the group consistingof AlN and InN, and p-type gallium nitride and/or n-type gallium nitridesingle crystal grains may be those in which such mixed-crystal galliumnitride is doped with a p-type dopant or an n-type dopant. For example,doping Al_(x)Ga_(1-x)N, which is a mixed crystal of gallium nitride andAlN, with Mg makes it possible to provide a p-type substrate, and dopingAl_(x)Ga_(1-x)N with Si makes it possible to provide an n-typesubstrate. When the self-supporting substrate is used as a lightemitting functional layer of a light emitting device, forming galliumnitride into a mixed crystal with AlN widens the band gap and makes itpossible to shift the emission wavelength toward the high energy side.Moreover, gallium nitride may be formed into a mixed crystal with InN,and this narrows the band gap and makes it possible to shift theemission wavelength toward the low energy side.

It is preferable that the self-supporting gallium nitride substrate hasa size of 50.8 mm (2 inches) or greater in diameter, more preferably 100mm (4 inches) or greater in diameter, and even more preferably 200 mm (8inches) or greater in diameter. The larger the self-supporting galliumnitride substrate is, the greater the number of producible devices is,and therefore a larger size is preferable from the viewpoint ofproduction cost, and is also preferable from the viewpoint of use insurface-light-emitting devices because the usable device area isenlarged so as to expand applications to surface-emitting lightings andthe like. Therefore, the upper limits of the area and size thereofshould not be specified. It is preferable that the self-supportinggallium nitride substrate is circular or substantially circular asviewed from above, but the shape is not limited thereto. When theself-supporting gallium nitride substrate is not circular orsubstantially circular, the area is preferably 2026 mm² or greater, morepreferably 7850 mm² or greater, and even more preferably 31400 mm² orgreater. For applications that do not require a large area, the area maybe smaller than the above range such as 50.8 mm (2 inches) or less indiameter, or 2026 mm² or less in terms of area. The thickness of theself-supporting gallium nitride substrate needs to be capable ofimparting self-supporting properties to the substrate, and is thuspreferably 20 μm or greater, more preferably 100 μm or greater, and evenmore preferably 300 μm or greater. Although the upper limit of thethickness of the self-supporting gallium nitride substrate should not bespecified, the thickness is realistically 3000 μm or less from theviewpoint of production cost.

The aspect ratio T/D_(T), which is defined as the ratio of the thicknessT of the self-supporting gallium nitride substrate to thecross-sectional average diameter D_(T) at the outermost surface ofgallium nitride-based single crystal grains exposed at the top surfaceof the self-supporting gallium nitride substrate, is preferably 0.7 orgreater, more preferably 1.0 or greater, and even more preferably 3.0 orgreater. This aspect ratio is preferable from the viewpoint ofincreasing luminous efficiency in the case of LEDs. As for the cause ofincreased luminous efficiency, it is considered that grains with a highaspect ratio result in a low defect density in gallium nitride,increased light extraction efficiency, and so on, but details thereofare not clear.

As described so far, from the viewpoint of increasing luminousefficiency, it is preferable that (1) a light emitting functional layeris produced on the top surface side of the self-supporting substrate(the side opposite to the side that was in contact with thebase-substrate oriented polycrystalline sintered body duringproduction), (2) the ratio D_(T)/D_(B), which is the ratio of thecross-sectional average diameter D_(T) of the substrate top surface tothe cross-sectional average diameter D_(B) of the substrate bottomsurface, is at a suitable value, (3) the cross-sectional averagediameter at the substrate outermost surface of grains constituting theself-supporting substrate is large, and (4) the aspect ratio T/D_(T) ofgrains constituting the self-supporting substrate is large. From theviewpoints (3) and (4) above, it is preferable that the cross-sectionalaverage diameter is large and the aspect ratio is large, or in otherwords, a gallium nitride crystal that has a large cross-sectionalaverage diameter on the top surface side of the substrate and a largethickness is preferable. Moreover, from the self-supporting viewpoint,the thickness of the self-supporting gallium nitride substrate ispreferably 20 μm or greater, more preferably 100 μm or greater, and evenmore preferably 300 μm or greater. However, as described above, a largethickness of a gallium nitride crystal is not preferable from the costviewpoint, and as long as the substrate is self-supporting, a lowerthickness is preferable. That is, the thickness of the self-supportinggallium nitride substrate is realistically 3000 μm or less, preferably600 μm or less, and preferably 300 μm or less. Therefore, the thicknessis preferably about 50 to 500 μm and more preferably about 50 to 300 μmfrom the viewpoint of allowing the substrate to be self-supporting andincreasing the luminous efficiency as well as from the viewpoint ofcost.

Manufacturing Method

The self-supporting gallium nitride substrate of the present inventioncan be manufactured by (1) providing an oriented polycrystallinesintered body, (2) forming a seed crystal layer composed of galliumnitride on the oriented polycrystalline sintered body so that the seedcrystal layer has crystal orientation mostly in conformity with thecrystal orientation of the oriented polycrystalline sintered body, (3)forming a layer with a thickness of 20 μm or greater composed of galliumnitride-based crystals on the seed crystal layer so that the layer hascrystal orientation mostly in conformity with the crystal orientation ofthe seed crystal layer, and (4) removing the oriented polycrystallinesintered body to obtain the self-supporting gallium nitride substrate.

(1) Oriented Polycrystalline Sintered Body

An oriented polycrystalline sintered body is provided as a basesubstrate for producing a self-supporting gallium nitride substrate.Although the composition of the oriented polycrystalline sintered bodyis not particularly limited, the oriented polycrystalline sintered bodyis preferably one selected from an oriented polycrystalline aluminasintered body, an oriented polycrystalline zinc oxide sintered body, andan oriented polycrystalline aluminum nitride sintered body. The orientedpolycrystalline sintered body can be efficiently manufactured throughforming and firing using a commercially available plate-shaped powder,and thus is not only able to be produced at low cost but also suitablefor having a large area due to ease in forming. According to theinventors' findings, by using an oriented polycrystalline sintered bodyas a base substrate and allowing a plurality of semiconductor singlecrystal grains to grow thereon, it is possible to manufacture aself-supporting gallium nitride substrate that is suitable formanufacturing large-area light emitting devices at low cost. As aresult, the self-supporting gallium nitride substrate is extremelysuitable for manufacturing large-area light emitting devices at lowcost.

The oriented polycrystalline sintered body is composed of a sinteredbody that contains numerous single crystal grains which are to someextent or highly oriented in a certain direction. The use of apolycrystalline sintered body oriented in this way makes it possible toproduce a self-supporting gallium nitride substrate having crystalorientation that is mostly aligned in the approximately normaldirection, and when a gallium nitride-based material is formed on theself-supporting gallium nitride substrate by epitaxial growth or crystalgrowth similar thereto, a state in which crystal orientation is mostlyaligned in the approximately normal direction is achieved. Accordingly,the use of such a highly oriented self-supporting gallium nitridesubstrate as a substrate for a light emitting device makes it possibleto form a light emitting functional layer that is similarly in a statein which its crystal orientation is mostly aligned in the approximatelynormal direction and makes it possible to achieve high luminousefficiency identical to that obtained when a single crystal substrate isused. Alternatively, even when this highly oriented self-supportinggallium nitride substrate is used as a light emitting functional layerof a light emitting device, it is possible to achieve high luminousefficiency identical to that obtained when a single crystal substrate isused. In any case, in order to produce such a highly orientedself-supporting gallium nitride substrate, an oriented polycrystallinesintered body needs to be used as a base substrate. Although it ispreferable that the oriented polycrystalline sintered body istransparent or translucent, the sintered body is not limited in thisrespect. In the case where the sintered body is transparent ortranslucent, a technique such as laser lift-off can be used for removingthe oriented polycrystalline plate. In addition to commonly usedpressureless sintering methods in which an air atmosphere furnace, anitrogen atmosphere furnace, a hydrogen atmosphere furnace, or the likeis used, pressure sintering methods such as hot isostatic pressing(HIP), hot pressing (HP), and spark plasma sintering (SPS), andcombination thereof can be used as production methods for obtaining theoriented polycrystalline sintered body.

The oriented polycrystalline sintered body preferably has a size of 50.8mm (2 inches) or greater in diameter, more preferably 100 mm (4 inches)or greater in diameter, and even more preferably 200 mm (8 inches) orgreater in diameter. The larger the oriented polycrystalline sinteredbody is, the larger the area of the producible self-supporting galliumnitride substrate is and thus the more the number of producible lightemitting devices is, and therefore a larger size is preferable from theviewpoint of production cost. Moreover, a larger size is also preferablefrom the viewpoint of use in surface-light-emitting devices because theusable device area is enlarged so as to expand applications tosurface-emitting lightings and the like, and therefore, the upper limitsof the area and size thereof should not be specified. It is preferablethat the self-supporting gallium nitride substrate is circular orsubstantially circular as viewed from above, but the shape is notlimited thereto. In the case where the self-supporting gallium nitridesubstrate is not circular or substantially circular, the area ispreferably 2026 mm² or greater, more preferably 7850 mm² or greater, andeven more preferably 31400 mm² or greater. For applications that do notrequire a large area, the area may be smaller than the above range suchas 50.8 mm (2 inches) or less in diameter, or 2026 mm² or less in termsof area. Although the thickness of the oriented polycrystalline sinteredbody is not limited as long as it is self-supporting, an excessivelylarge thickness is not preferable from the viewpoint of production cost.Therefore, the thickness is preferably 20 μm or greater, more preferably100 μm or greater, and even more preferably 100 to 1000 μm. Meanwhile,In the case of forming gallium nitride into a film, the entire substratemay warp due to stress resulting from the difference between the thermalexpansions of alumina and gallium nitride, thus adversely affecting thesubsequent process. Although stress varies according to, for example,the method for forming a gallium nitride film, film formationconditions, materials of the oriented polycrystalline sintered body,film thickness, and substrate diameter, a thick oriented polycrystallinesintered body may be used as a base substrate as one technique ofsuppressing warpage due to stress. For example, in the case of producinga self-supporting gallium nitride substrate having a diameter of 50.8 mm(2 inches) and a thickness of 300 μm using an oriented polycrystallinealumina sintered body as a base oriented polycrystalline sintered body,the thickness of the oriented polycrystalline alumina sintered body maybe 900 μm or greater, 1300 μm or greater, or 2000 μm or greater. In thisway, the thickness of the oriented polycrystalline sintered body may besuitably selected in consideration of, for example, the viewpoint ofproduction cost and the viewpoint of suppressing warpage.

The average grain diameter at the plate surface of grains constitutingthe oriented polycrystalline sintered body is preferably 0.3 to 1000 μm,more preferably 3 to 1000 μm, even more preferably 10 μm to 200 μm, andparticularly preferably 14 μm to 200 μm. Alternatively, as describedabove, in the case of considering that the cross-sectional averagediameter of semiconductor single crystal grains at the outermost surfaceof the self-supporting gallium nitride substrate is made larger than thecross-sectional average diameter at the bottom surface of theself-supporting substrate, the sintered grain diameter at the platesurface of grains constituting the oriented polycrystalline sinteredbody is preferably 10 μm to 100 μm and more preferably 14 μm to 70 μm.The overall average grain diameter of the oriented polycrystallinesintered body correlates with the average grain diameter at the platesurface, and when the diameter is within these ranges, the sintered bodyis excellent in terms of mechanical strength and is easy to be handled.Moreover, when a light emitting device is produced by forming a lightemitting functional layer in the upper part and/or the interior of aself-supporting gallium nitride substrate produced using an orientedpolycrystalline sintered body, the luminous efficiency of the lightemitting functional layer is also excellent. The average grain diameterat the plate surface of sintered body grains in the present invention ismeasured by the following method. That is, the plate surface of aplate-shaped sintered body is polished, and an image is taken with ascanning electron microscope. The visual field range is determined in away such that when straight lines are diagonally drawn on the obtainedimage, each straight line crosses 10 to 30 grains. The average graindiameter at the plate surface is determined by diagonally drawing twostraight lines on the obtained image, taking the average of the linesegment lengths inside all grains crossed by the straight lines, andmultiplying the average by 1.5. When the boundary of sintered bodygrains cannot be clearly determined on the scanning microscope image ofthe plate surface, the above evaluation may be carried out afterperforming processing to emphasize the boundary by thermal etching (forexample, for 45 minutes at 1550° C.) or chemical etching.

A particularly preferable oriented polycrystalline sintered body is anoriented polycrystalline alumina sintered body. Alumina is aluminumoxide (Al₂O₃) and is typically α-alumina having the same corundum-typestructure as single crystal sapphire, and the oriented polycrystallinealumina sintered body is a solid in which a countless number of aluminacrystal grains in an oriented state are bonded to each other bysintering. Alumina crystal grains contain alumina and may contain adopant and an inevitable impurity as other elements, or that may becomposed of alumina and an inevitable impurity. The orientedpolycrystalline alumina sintered body may contain an additive as asintering aid in a grain boundary phase. Although the orientedpolycrystalline alumina sintered body may also contain another phase oranother element as described above in addition to alumina crystalgrains, preferably the oriented polycrystalline alumina sintered body iscomposed of alumina crystal grains and an inevitable impurity. Theoriented plane of the oriented polycrystalline alumina sintered body isnot particularly limited and may be a c-plane, an a-plane, an r-plane,an m-plane, or the like.

The direction in which crystals are oriented in the orientedpolycrystalline alumina sintered body is not particularly limited, andit may be the direction of a c-plane, an a-plane, an r-plane, anm-plane, or the like, and from the viewpoint of lattice constantmatching with the self-supporting gallium nitride substrate, it ispreferable that crystals are oriented along the c-plane. As for thedegree of orientation, for example, the degree of orientation at theplate surface is preferably 50% or greater, more preferably 65% orgreater, even more preferably 75% or greater, particularly preferably85% or greater, particularly more preferably 90% or greater, and mostpreferably 95% or greater. The degree of orientation can be determinedby measuring an XRD profile through irradiating the plate surface ofplate-shaped alumina with X rays using an XRD apparatus (such asRINT-TTR III manufactured by Rigaku Corporation) and calculatingaccording to the formulae below.

$\begin{matrix}{{{{Degree}\mspace{14mu} {of}\mspace{14mu} {{Orientation}\mspace{11mu}\lbrack\%\rbrack}} = {\frac{p - p_{0}}{1 - p_{0}} \times 100}}{p_{0} = \frac{I_{0}({hkl})}{\sum{I_{0}({hkl})}}}{p = \frac{I_{s}({hkl})}{\sum{I_{s}({hkl})}}}} & \lbrack {{Mathematical}\mspace{14mu} {Formula}\mspace{14mu} 1} \rbrack\end{matrix}$

where I₀(hkl) and I_(S)(hkl) are the integral values)(2θ=20−70° of thediffraction intensities from the (hkl) planes in ICDD No. 461212 and asample, respectively.

The crystallinity of constitutive grains of the self-supporting galliumnitride substrate tends to be high, and the density of defects such asdislocation can be kept low. Accordingly, it is considered that, in someapplications such as light emitting devices, it is even possible to usethe self-supporting gallium nitride substrate more preferably than agallium nitride single crystal substrate. For example, when a functionallayer is produced on the self-supporting gallium nitride substrate byepitaxial growth, the functional layer grows mostly in conformity withthe base self-supporting gallium nitride substrate and becomes anaggregate of columnar structures. In epitaxial growth, the crystalquality of the base is inherited, and therefore it is possible to obtainhigh crystal quality in each domain of columnar structures constitutingthe functional layer. Although the reason why the crystal grainsconstituting the self-supporting gallium nitride substrate has a lowdefect density is not clear, it is presumed that among the latticedefects occurring during the initial stage of production of theself-supporting gallium nitride substrate, those that develop with tilttoward the horizontal direction are absorbed by the grain boundary asgrowth progresses, and thus disappear.

From the viewpoint of lowering the density of defects such asdislocation contained in the self-supporting gallium nitride substrate,it is more preferable that when producing the self-supporting galliumnitride substrate, some or all grains constituting the outermost surfaceof the oriented polycrystalline sintered body that serves as a basesubstrate are arranged so as to be slightly tilted randomly from acertain direction (such as a c-plane, an a-plane, or a like referencedirection). Approximately all or a certain amount of the tilted grainsmay be tilted at an approximately constant angle, or may be tilted atvarious angles and/or in various directions so as to have a distributionwithin a certain range (preferably 0.01 to 20°). Moreover, tilted grainsand non-tilted grains may be intermixed in a desired proportion.Alternatively, the plate surface of the oriented polycrystalline aluminasintered body may be obliquely polished relative to the reference planeto allow the exposed surface of the grains to be tilted in a certaindirection, or a plane slightly tilted from the reference direction ofthe grains at the outermost surface may be exposed by processing into awave-like form or the like. In any of the above cases, it is preferablethat some or all alumina single crystal grains constituting theoutermost surface of the oriented polycrystalline sintered body orientedin a reference direction such as a c-plane or an a-plane are arranged ina tilted manner so that their reference orientation is shifted within arange of 0.5 to 20° from the normal direction of the substrate.

The oriented polycrystalline alumina sintered body can be manufacturedby forming and sintering, using a plate-shaped alumina powder as a rawmaterial. A plate-shaped alumina powder is sold in the market and iscommercially available. Although the type and the shape of theplate-shaped alumina powder are not particularly limited as long as adense oriented polycrystalline alumina sintered body can be obtained,the average grain diameter may be 0.4 to 15 μm and the thickness may be0.05 to 1 μm, and a mixture of two or more raw materials havingdifferent average grain diameters within this range may be used.Preferably, a plate-shaped alumina powder can be formed into an orientedgreen body by a technique that uses shearing force. Preferable examplesof techniques that use shearing force include tape casting, extrusionmolding, doctor blade method, and any combination of these. In theorientation technique that uses shearing force, it is preferable, in anytechnique exemplified above, that additives such as a binder, aplasticizer, a dispersing agent, and a dispersion medium are suitablyadded to the plate-shaped alumina powder to form a slurry, and theslurry is allowed to pass through a slit-shaped narrow discharge port todischarge and form the slurry into a sheet on a base. The slit width ofthe discharge port is preferably 10 to 400 μm. The amount of thedispersion medium is adjusted so that the viscosity of the slurry ispreferably 5000 to 100000 cP and more preferably 20000 to 60000 cP. Thethickness of the oriented green body formed into a sheet is preferably 5to 500 μm and more preferably 10 to 200 μm. It is preferable thatnumerous pieces of this oriented green body that has been formed into asheet are stacked on top of the other to form a precursor laminatehaving a desired thickness, and press molding is performed on thisprecursor laminate. This press molding can be preferably performed bypacking the precursor laminate in a vacuum pack or the like andsubjecting it to isostatic pressing in hot water at 50 to 95° C. under apressure of 10 to 2000 kgf/cm². Moreover, the oriented green body thathas been formed into a sheet, or the precursor laminate, may besubjected to processing by a roll press method (such as a heating rollpress or a calender roll). Moreover, when extrusion molding is used, theflow channel within a metal mold may be designed so that after passingthrough a narrow discharge port within the metal mold, sheets of thegreen body are integrated into a single body within the metal mold, andthe green body is ejected in a laminated state. It is preferable todegrease the resulting green body in accordance with known conditions.The oriented green body obtained in the above manner is fired by, inaddition to ordinal pressureless firing using an air atmosphere furnace,a nitrogen atmosphere furnace, a hydrogen atmosphere furnace, or thelike, pressure sintering methods such as hot isostatic pressing (HIP),hot pressing (HP), and spark plasma sintering (SPS), and combinationthereof, to form an alumina sintered body containing oriented aluminacrystal grains. Although the firing temperature and the firing time inthe above firing depend on the firing method, the firing temperature maybe 1000 to 1950° C., preferably 1100 to 1900° C., and more preferably1500 to 1800° C., and the firing time may be 1 minute to 10 hours andpreferably 30 minutes to 5 hours. Form the viewpoint of promotingdensification, firing is preferably performed through a first firingstep of firing the green body with a hot pressing under conditions of1500 to 1800° C., 2 to 5 hours, and a surface pressure of 100 to 200kgf/cm², and a second firing step of re-firing the resulting sinteredbody with hot isostatic pressing (HIP) under conditions of 1500 to 1800°C., 30 minutes to 5 hours, and a gas pressure of 1000 to 2000 kgf/cm².Although the firing time at the aforementioned firing temperature is notparticularly limited, it is preferably 1 to 10 hours and more preferably2 to 5 hours. In the case of imparting translucency, a preferableexample is a method in which a high-purity plate-shaped alumina powderis used as a raw material and fired in an air atmosphere furnace, ahydrogen atmosphere furnace, a nitrogen atmosphere furnace, or the likeat 1100 to 1800° C. for 1 minute to 10 hours. A method may be used inwhich the resulting sintered body is re-fired with hot isostaticpressing (HIP) under conditions of 1200 to 1400° C. or 1400 to 1950° C.,30 minutes to 5 hours, and a gas pressure of 300 to 2000 kgf/cm². Thefewer the grain boundary phases are, the more preferable it is, andtherefore it is preferable that the plate-shaped alumina powder has highpurity, and more preferably the purity is 98% or higher, even morepreferably 99% or higher, particularly preferably 99.9% or higher, andmost preferably 99.99% or higher. The firing conditions are not limitedto those described above, and the second firing step with, for example,hot isostatic pressing (HIP) may be omitted as long as densification andhigh orientation can be simultaneously achieved. Moreover, an extremelysmall amount of additive may be added to the raw material as a sinteringaid. Addition of a sintering aid, although it is contradictory toreducing the amount of grain boundary phase, is for reducing pores thatare one of the factors causing scattering of light and, as a result,increasing translucency. Examples of such sintering aids include atleast one selected from oxides such as MgO, ZrO₂, Y₂O₃, CaO, SiO₂, TiO₂,Fe₂O₃, Mn₂O₃, and La₂O₃, fluorides such as AlF₃, MgF₂, and YbF₃, and thelike. Among these, MgO, CaO, SiO₂, and La₂O₃ are preferable, and MgO isparticularly preferable. However, from the translucency viewpoint, theamount of additive should be limited to the smallest necessary level,and is preferably 5000 ppm or less, more preferably 1000 ppm or less,and even more preferably 700 ppm or less.

Moreover, the oriented polycrystalline alumina sintered body can beproduced also by forming and sintering, using a mixed powder in which aplate-shaped alumina powder is suitably added to a fine alumina powderand/or transition alumina powder as a raw material. In this productionmethod, crystal growth and densification occur through a so-called TGG(Templated Grain Growth) process in which the plate-shaped aluminapowder serves as a seed crystal (template), the fine alumina powderand/or transition alumina powder serves as a matrix, and the templategrows homoepitaxially while incorporating the matrix. As for the graindiameters of the plate-shaped alumina grains serving as a template andof the matrix, the larger the grain diameter ratio thereof is, the moreeasily the grains grow. For example, when the average grain diameter ofthe template is 0.5 to 15 μm, the average grain diameter of the matrixis preferably 0.4 μm or less, more preferably 0.2 μm or less, and evenmore preferably 0.1 μm or less. Although the mixing ratio of thetemplate and the matrix varies according to the grain diameter ratio,firing conditions, and presence or absence of an additive, thetemplate/matrix ratio may be 50/50 to 1/99 wt % when, for example, aplate-shaped alumina powder having an average grain diameter of 2 μm isused for the template and a fine alumina powder having an average graindiameter of 0.1 μm is used for the matrix. From the viewpoint ofpromoting densification, at least one selected from oxides such as MgO,ZrO₂, Y₂O₃, CaO, SiO₂, TiO₂, Fe₂O₃, Mn₂O₃, and La₂O₃, fluorides such asAlF₃, Mg F₂, and YbF₃, and the like may be added as a sintering aid, andMgO, CaO, SiO₂, and La₂O₃ are preferable, and MgO is particularlypreferable. In such a technique as well, a high-quality orientedpolycrystalline alumina sintered body can be obtained by theaforementioned pressure sintering methods such as hot isostatic pressing(HIP), hot pressing (HP), and spark plasma sintering (SPS), andcombination thereof, in addition to ordinal pressureless firing using anair atmosphere furnace, a nitrogen atmosphere furnace, a hydrogenatmosphere furnace, or the like.

The alumina sintered body obtained in this way is a polycrystallinealumina sintered body oriented in the direction of a desired plane suchas a c-plane in accordance with the type of the aforementionedraw-material plate-shaped alumina powder. It is preferable that theoriented polycrystalline alumina sintered body obtained in this way isground with grinding wheel to flatten the plate surface, and then theplate surface is smoothed by lapping using diamond abrasive grains toobtain an oriented alumina substrate.

(2) Formation of Seed Crystal Layer

A seed crystal layer composed of gallium nitride is formed on theoriented polycrystalline sintered body so that the seed crystal layerhas crystal orientation that is mostly in conformity with the crystalorientation of the oriented polycrystalline sintered body. The phrase“formed so that the seed crystal layer has crystal orientation that ismostly in conformity with the crystal orientation of the orientedpolycrystalline sintered body” means a structure resulting from crystalgrowth influenced by the crystal orientation of the orientedpolycrystalline sintered body, and it is not necessarily limited to astructure in which grains are grown completely in conformity with thecrystal orientation of the oriented polycrystalline sintered body andalso includes a structure in which grains grow in a crystal orientationdifferent from that of the oriented polycrystalline sintered body. Amethod for producing a seed crystal layer is not particularly limited,and preferable examples include vapor phase methods such as MOCVD (metalorganic chemical vapor deposition), MBE (molecular beam epitaxy), HVPE(halide vapor phase epitaxy), and sputtering, liquid phase methods suchas Na flux method, ammonothermal method, hydrothermal method, andsol-gel method, powder methods that utilize solid phase growth ofpowder, and combinations of these. For example, formation of a seedcrystal layer by MOCVD is preferably performed by depositing a 20-50 nmthick low-temperature GaN layer at 450 to 550° C. and laminating a GaNfilm having a thickness of 2 to 4 μm at 1000 to 1200° C.

(3) Formation of Gallium Nitride-Based Crystal Layer

A layer with a thickness of 20 μm or greater composed of galliumnitride-based crystals is formed on the seed crystal layer so that thelayer has crystal orientation that is mostly in conformity with thecrystal orientation of the seed crystal layer. A method for forming alayer composed of gallium nitride-based crystals is not particularlylimited as long as the layer has crystal orientation that is mostly inconformity with the crystal orientation of the oriented polycrystallinesintered body and/or the seed crystal layer. Preferable examples includevapor phase methods such as MOCVD and HYPE, liquid phase methods such asNa flux method, ammonothermal method, hydrothermal method, and sol-gelmethod, powder methods that utilize solid phase growth of powder, andcombinations of these, and it is particularly preferable that the layeris formed by Na flux method. A highly crystalline, thick gallium nitridecrystal layer can be efficiently produced on the seed crystal layer byNa flux method. It is preferable that formation of a galliumnitride-based crystal layer by Na flux method is performed by filling acrucible containing a seed crystal substrate with a melt compositioncontaining metal Ga and metal Na and optionally a dopant (e.g., ann-type dopant such as germanium (Ge), silicon (Si), or oxygen (O), or ap-type dopant such as beryllium (Be), magnesium (Mg), calcium (Ca),strontium (Sr), zinc (Zn), or cadmium (Cd)), increasing the temperatureand the pressure to 830 to 910° C. and 3.5 to 4.5 MPa in a nitrogenatmosphere, and then rotating the crucible while retaining thetemperature and the pressure. Although the retention time depends on theintended film thickness, it may be about 10 to 100 hours. Moreover, itis preferable that the gallium nitride crystals obtained by Na fluxmethod are ground with grinding wheel to flatten the plate surface, andthen the plate surface is smoothed by lapping using diamond abrasivegrains.

(4) Removal of Oriented Polycrystalline Sintered Body

The self-supporting gallium nitride substrate can be obtained byremoving the oriented polycrystalline sintered body. A method forremoving the oriented polycrystalline sintered body is not particularlylimited, and examples include grinding, chemical etching, interfacialheating by laser irradiation from the oriented sintered body side (laserlift-off), spontaneous separation utilizing a difference in thermalexpansion when the temperature is increased, and the like.

Light Emitting Device and Manufacturing Method Therefor

A high-quality light emitting device can be produced using theself-supporting gallium nitride substrate of the present inventiondescribed above. Neither the structure of the light emitting deviceincluding the self-supporting gallium nitride substrate of the presentinvention nor the production method therefor is particularly limited.Typically, it is preferable that the light emitting device is producedby providing a light emitting functional layer on the self-supportinggallium nitride substrate, and formation of this light emittingfunctional layer is performed by forming at least one layer composed ofa plurality of semiconductor single crystal grains, wherein the at leastone layer has a single crystal structure in the approximately normaldirection so that the at least one layer has crystal orientation mostlyin conformity with the crystal orientation of the gallium nitridesubstrate. The self-supporting gallium nitride substrate may be used asa component or a layer other than a base material, such as an electrode(which may be a p-type electrode or an n-type electrode), a p-type,layer, an n-type layer, or the like, to produce a light emitting device.The device size is not particularly limited, and the device may be asmall device having no greater than 5 mm×5 mm or may be asurface-emitting device having no less than 10 cm×10 cm.

FIG. 1 schematically shows the layer structure of a light emittingdevice according to one embodiment of the present invention. A lightemitting device 10 shown in FIG. 1 includes a self-supporting galliumnitride substrate 12 and a light emitting functional layer 14 formed onthis substrate. The light emitting functional layer 14 has at least onelayer composed of a plurality of semiconductor single crystal grains,wherein the at least one layer has a single crystal structure in theapproximately normal direction. This light emitting functional layer 14emits light based on the principle of light emitting devices such asLEDs by suitably providing electrodes and the like and applying voltage.In particular, by using the self-supporting gallium nitride substrate 12of the present invention, it can also be expected to obtain a lightemitting device having luminous efficiency equivalent to that when agallium nitride single crystal substrate is used, and a significant costreduction can be achieved. Moreover, by using gallium nitride substrateprovided with electroconductivity by introducing a p-type or n-typedopant, it is possible to achieve a light emitting device having avertical structure, and it is thereby possible to increase luminance. Inaddition, a large-area surface emitting device can be achieved at lowcost.

The light emitting functional layer 14 is formed on the substrate 12.The light emitting functional layer 14 may be formed on the entiresurface or a part of the substrate 12, or when a buffer layer asdescribed later is formed on the substrate 12, the light emittingfunctional layer 14 may be formed on the entire surface or a part of thebuffer layer in the case. The light emitting functional layer 14 has atleast one layer composed of a plurality of semiconductor single crystalgrains wherein the at least one layer has a single crystal structure inthe approximately normal direction, and can take a variety of knownlayer configurations that bring about light emission based on theprinciple of light emitting devices represented by LEDs by suitablyproviding an electrode and/or a phosphor and applying voltage.Therefore, the light emitting functional layer 14 may emit visible lightsuch as blue or red, or may emit ultraviolet light with or withoutvisible light. It is preferable that the light emitting functional layer14 constitutes at least part of a light emitting device that utilizes ap-n junction, and this p-n junction may include an active layer 14 bbetween a p-type layer 14 a and an n-type layer 14 c as shown in FIG. 1.At this time, a double heterojunction or a single heterojunction(hereinafter collectively referred to as a heterojunction) in which alayer having a smaller band gap than the p-type layer and/or the n-typelayer is used as the active layer may be used. Moreover, as one form ofp-type layer/active layer/n-type layer, a quantum well structure inwhich the thickness of the active layer is made small can be adopted.Needless to say, in order to obtain a quantum well, a doubleheterojunction should be employed in which the band gap of the activelayer is made smaller than those of the p-type layer and the n-typelayer. Moreover, a multiple quantum well structure (MQW) may be used inwhich a large number of such quantum well structures are stacked.Adopting these structures makes it possible to increase luminousefficiency in comparison to a p-n junction. In this way, it ispreferable that the light emitting functional layer 14 includes a p-njunction and/or a heterojunction and/or a quantum well junction having alight emitting function.

Therefore, the at least one layer constituting the light emittingfunctional layer 14 can include at least one selected from the groupconsisting of an n-type layer doped with an n-type dopant, a p-typelayer doped with a p-type dopant, and an active layer. The n-type layer,the p-type layer, and the active layer (if present) may be composed ofmaterials whose main components are the same or different to each other.

The material of each layer constituting the light emitting functionallayer 14 is not particularly limited as long as it grows mostly inconformity with the crystal orientation of the self-supporting galliumnitride substrate and has a light emitting function, and is preferablycomposed of a material whose main component is at least one selectedfrom gallium nitride (GaN)-based materials, zinc oxide (ZnO)-basedmaterials, and aluminum nitride (AlN)-based materials, and may suitablycontain a dopant for controlling it to be a p-type or an n-type. Aparticularly preferable material is a gallium nitride (GaN)-basedmaterial, which is the same type of material as the self-supportinggallium nitride substrate. Moreover, the material constituting the lightemitting functional layer 14 may be a mixed crystal in which AlN, InN,or the like forms a solid solution with GaN, for controlling the bandgap thereof. Moreover, as described in the last paragraph, the lightemitting functional layer 14 may be a heterojunction composed ofmultiple types of material systems. For example, a gallium nitride(GaN)-based material may be used for the p-type layer, and a zinc oxide(ZnO)-based material may be used for the n-type layer. Moreover, a zincoxide (ZnO)-based material may be used for the p-type layer, a galliumnitride (GaN)-based material may be used for the active layer as well asthe n-type layer, and there is not a particular limitation to materialcombinations.

The each layer constituting the light emitting functional layer 14 iscomposed of a plurality of semiconductor single crystal grains, whereinthe layer has a single crystal structure in the approximately normaldirection. That is, each layer is composed of a plurality ofsemiconductor single crystal grains connected two-dimensionally in thedirection of a horizontal plane, and, therefore, has a single crystalstructure in the approximately normal direction. Therefore, althougheach layer of the light emitting functional layer 14 is not a singlecrystal as a whole, it has a single crystal structure in terms of localdomains, and can therefore have sufficiently high crystallinity forensuring a light emitting function. Preferably, the semiconductor singlecrystal grains constituting the respective layers of the light emittingfunctional layer 14 have a structure in which grains are grown mostly inconformity with the crystal orientation of the self-supporting galliumnitride substrate, which is the substrate 12. The “structure in whichgrains are grown mostly in conformity with the crystal orientation ofthe self-supporting gallium nitride substrate” means a structureresulting from crystal growth influenced by the crystal orientation ofthe self-supporting gallium nitride substrate, and it is not necessarilylimited to a structure in which grains are grown completely inconformity with the crystal orientation of the self-supporting galliumnitride substrate, and may be a structure in which grains are grown, tosome extent, in conformity with the crystal orientation of theself-supporting gallium nitride substrate as long as a desired lightemitting function can be ensured. That is, this structure also includesa structure in which grains are grown in crystal orientation differentfrom that of the oriented polycrystalline sintered body. In this sense,the expression “structure in which grains are grown mostly in conformitywith crystal orientation” can be paraphrased as “structure in whichgrains are grown in a manner mostly derived from crystal orientation”.Therefore, such crystal growth is preferably epitaxial growth, but it isnot limited thereto, and may take a variety of similar crystal growthforms. In particular, when the layers respectively constituting then-type layer, the active layer, the p-type layer, and the like grow inthe same crystal orientation as the self-supporting gallium nitridesubstrate, the structure is such that the crystal orientation from theself-supporting gallium nitride substrate to each layer of the lightemitting functional layer is mostly aligned with respect to theapproximately normal direction, and favorable light emitting propertiescan be obtained. That is, when the light emitting functional layer 14also grows mostly in conformity with the crystal orientation of theself-supporting gallium nitride substrate 12, the orientation is mostlyuniform in the direction perpendicular to the substrate. Accordingly, astate identical to a single crystal is attained in the normal direction,and by using a self-supporting gallium nitride substrate to which ann-type dopant is added, a light emitting device having a verticalstructure can be formed in which the self-supporting gallium nitridesubstrate is used as a cathode, while by using a self-supporting galliumnitride substrate to which a p-type dopant is added, a light emittingdevice having a vertical structure can be formed in which theself-supporting gallium nitride substrate is used as an anode.

When at least the layers, such as the n-type layer, the active layer,and the p-type layer, constituting the light emitting functional layer14 grow in the same crystal orientation, each layer is observed as asingle crystal when viewed in the normal direction, and thus it is alsopossible to recognize it as an aggregate of semiconductor single crystalgrains having a columnar structure in which a grain boundary is observedwhen the cross section in the direction of a horizontal plane is viewed.Here, the “columnar structure” does not mean only a typical verticallylong columnar shape, and is defined as having a meaning encompassingvarious shapes such as a horizontally long shape, a trapezoidal shape,and an inverted trapezoidal shape. As described above, each layer mayhave a structure in which grains are grown, to some extent, inconformity with the crystal orientation of the self-supporting galliumnitride substrate, and does not necessarily need to have a columnarstructure in a strict sense. As described above, the growth of galliumnitride single crystal grains due to the influence of the crystalorientation of the self-supporting gallium nitride substrate, which isthe substrate 12, is considered to be the cause of the columnarstructure. Therefore, the average grain diameter at the cross section(hereinafter referred to as a cross-sectional average diameter) ofsemiconductor single crystal grains that can also be called columnarstructures is considered to depend on not only the conditions of filmformation but also the average grain diameter at the plate surface ofthe self-supporting gallium nitride substrate. The interface of columnarstructures constituting the light emitting functional layer influencesluminous efficiency and emission wavelength, and the presence of grainboundaries impairs light transmittance in the cross-sectional directionand causes light to be scattered or reflected. Therefore, in the case ofa structure from which light is extracted in the normal direction, aluminance increasing effect due to scattered light from grain boundariesis also expected.

Crystallinity at the interface between columnar structures constitutingthe light emitting functional layer 14 is low, and therefore there is apossibility that the luminous efficiency deteriorates, the emissionwavelength changes, and the emission wavelength broadens. Therefore, alarger cross-sectional average diameter of the columnar structures ispreferable. Preferably, the cross-sectional average diameter of thesemiconductor single crystal grains at the outermost surface of thelight emitting functional layer 14 is 0.3 μm or greater, more preferably3 μm or greater, even more preferably 20 μm or greater, particularlypreferably 50 μm or greater, and most preferably 70 μm or greater.Although the upper limit of this cross-sectional average diameter is notparticularly defined, it is realistically 1000 μm or less, morerealistically 500 μm or less, and even more realistically 200 μm orless. In order to produce semiconductor single crystal grains havingsuch a cross-sectional average diameter, it is desirable that thecross-sectional average diameter at the outermost surface of thesubstrate of gallium nitride-based single crystal grains that constitutethe self-supporting gallium nitride substrate is 0.3 μm to 1000 μm andmore desirably 3 μm or greater.

In the case where a material other than a gallium nitride (GaN)-basedmaterial is partially or entirely used for the light emitting functionallayer 14, a buffer layer may be provided between the self-supportinggallium nitride substrate 12 and the light emitting functional layer 14for inhibiting a reaction. The main component of such a buffer layer isnot particularly limited, and it is preferable that the buffer layer iscomposed of a material, the main component of which is at least oneselected from zinc oxide (ZnO)-based materials and aluminum nitride(AlN)-based materials, and may suitably contain a dopant for controllingit to be a p-type or an n-type.

It is preferable that each layer constituting the light emittingfunctional layer 14 is composed of a gallium nitride-based material. Forexample, an n-type gallium nitride layer and a p-type gallium nitridelayer may be grown in this order on the self-supporting gallium nitridesubstrate 12, or the order of stacking the p-type gallium nitride layerand the n-type gallium nitride layer may be inverse. Preferable examplesof p-type dopants used for the p-type gallium nitride layer include oneor more selected from the group consisting of beryllium (Be), magnesium(Mg), calcium (Ca), strontium (Sr), zinc (Zn), and cadmium (Cd).Preferable examples of n-type dopants used for the n-type galliumnitride layer include one or more selected from the group consisting ofsilicon (Si), germanium (Ge), tin (Sn), and oxygen (O). Moreover, thep-type gallium nitride layer and/or the n-type gallium nitride layer maybe composed of gallium nitride formed into a mixed crystal with acrystal of one or more selected from the group consisting of AlN andInN, and the p-type layer and/or the n-type layer may be thismixed-crystal gallium nitride doped with a p-type dopant or an n-typedopant. For example, doping Al_(x)Ga_(1-x)N, which is a mixed crystal ofgallium nitride and AlN, with Mg makes it possible to provide a p-typelayer, and doping Al_(x)Ga_(1-x)N with Si makes it possible to providean n-type layer. Forming gallium nitride into a mixed crystal with AlNwidens the band gap and makes it possible to shift the emissionwavelength toward the high energy side. Moreover, gallium nitride may beformed into a mixed crystal with InN, and this narrows the band gap andmakes it possible to shift the emission wavelength toward the low energyside. Between the p-type gallium nitride layer and the n-type galliumnitride layer, there may be an active layer composed of GaN or a mixedcrystal of GaN and one or more selected from the group consisting of AlNand InN, that has a smaller band gap than both layers. The active layerhas a structure that forms a double heterojunction with a p-type layerand an n-type layer, and a configuration in which this active layer ismade thin corresponds to the light emitting device having a quantum wellstructure, which is one form of a p-n junction, and luminous efficiencycan be further increased. Moreover, the active layer may be configuredto have a smaller band gap than either layer and be composed of GaN or amixed crystal of GaN and one or more selected from the group consistingof AlN and InN. Luminous efficiency can be further increased also bysuch a single heterojunction. The gallium nitride-based buffer layer maybe composed of non-doped GaN or n-type or p-type-doped GaN, may containAlN or InN having a close lattice constant, or may be a mixed crystalformed with GaN and one or more crystals selected from the groupconsisting of AlN and InN.

The light emitting functional layer 14 may be composed of a plurality ofmaterial systems selected from gallium nitride (GaN)-based materials,zinc oxide (ZnO)-based materials, and aluminum nitride (AlN)-basedmaterials. For example, a p-type gallium nitride layer and an n-typezinc oxide layer may be grown on the self-supporting gallium nitridesubstrate 12, and the order of stacking the p-type gallium nitride layerand the n-type zinc oxide layer may be inverse. In the case where theself-supporting gallium nitride substrate 12 is used as a part of thelight emitting functional layer 14, an n-type or p-type zinc oxide layermay be formed. Preferable examples of p-type dopants used for the p-typezinc oxide layer include one or more selected from the group consistingof nitrogen (N), phosphorus (P), arsenic (As), carbon (C), lithium (Li),sodium (Na), potassium (K), silver (Ag), and copper (Cu). Moreover,preferable examples of n-type dopants used for the n-type zinc oxidelayer include one or more selected from the group consisting of aluminum(Al), gallium (Ga), indium (In), boron (B), fluorine (F), chlorine (CI),bromine (Br), iodine (I), and silicon (Si).

A method for forming films of the light emitting functional layer 14 andthe buffer layer is not particularly limited as long as it allows growthmostly in conformity with the crystal orientation of the self-supportinggallium nitride substrate, and preferable examples include vapor phasemethods such as MOCVD, MBE, HVPE, and sputtering, liquid phase methodssuch as Na flux method, ammonothermal method, hydrothermal method, andsol-gel method, powder methods that utilize solid phase growth ofpowder, and combinations of these. For example, in the case where thelight emitting functional layer 14 composed of a gallium nitride-basedmaterial is produced using MOCVD, at least an organic metal gascontaining gallium (Ga) (such as trimethylgallium) and a gas containingat least nitrogen (N) (such as ammonia) as raw materials may be flownover a substrate to allow growth in, for example, an atmospherecontaining hydrogen, nitrogen, or both within a temperature range ofabout 300 to 1200° C. In this case, film formation may be performed bysuitably introducing an organic metal gas containing indium (In) oraluminum (Al) for band gap control as well as silicon (Si) or magnesium(Mg) as an n-type and p-type dopant (such as trimethylindium,trimethylaluminum, monosilane, disilane, andbis-cyclopentadienylmagnesium).

Moreover, in the case where materials other than gallium nitride-basedmaterials are used for the light emitting functional layer 14 and thebuffer layer, a film of a seed crystal layer may be formed on theself-supporting gallium nitride substrate. A method for forming a filmof the seed crystal layer and a material are not particularly limited aslong as crystal growth that is mostly in conformity with the crystalorientation is promoted. For example, when a zinc oxide-based materialis used for a part of or all of the light emitting functional layer 14,an extremely thin zinc oxide seed crystal may be produced using a vaporphase growth method such as MOCVD, MBE, HVPE, or sputtering.

An electrode layer 16 and/or a phosphor layer may be further provided onthe light emitting functional layer 14. As described above, since alight emitting device including the electroconductive self-supportinggallium nitride substrate 12 can have a vertical structure, an electrodelayer 18 can also be formed on the bottom surface of the self-supportinggallium nitride substrate 12 as shown in FIG. 1, but the self-supportinggallium nitride substrate 12 itself may be used as an electrode, and inthis case, it is preferable that an n-type dopant is added to theself-supporting gallium nitride substrate 12. The electrode layers 16and 18 may be composed of known electrode materials, and configuring theelectrode layer 16 on the light emitting functional layer 14 to be atransparent conductive film such as ITO or a metal electrode with alattice structure or the like having a high aperture ratio is preferablefor being able to increase the efficiency of extracting light producedin the light emitting functional layer 14.

When the light emitting functional layer 14 can emit ultraviolet light,a phosphor layer for converting ultraviolet light into visible light maybe provided on the outer side of the electrode layer. The phosphor layermay be a layer containing a known fluorescent component capable ofconverting ultraviolet rays into visible light, and is not particularlylimited. For example, preferable is such a configuration that afluorescent component that becomes excited by ultraviolet light andemits blue light, a fluorescent component that becomes excited byultraviolet light and emits blue to green light, and a fluorescentcomponent that becomes excited by ultraviolet light and emits red lightare allowed to be concomitantly present to obtain white light as a mixedcolor. Preferable combinations of such fluorescent components include(Ca,Sr)₅(PO₄)₃Cl:Eu, BaMgAl₁₀O₁₇:Eu and Mn, and Y₂O₃S:Eu, and it ispreferable to disperse these components in a resin such as siliconeresin to form a phosphor layer. Such fluorescent components are notlimited to components exemplified above, and other ultraviolet-excitedphosphors such as yttrium aluminum garnet (YAG), silicate phosphors, andoxynitride-based phosphors may be combined.

On the other hand, when the light emitting functional layer 14 can emitblue light, a phosphor layer for converting blue light into yellow lightmay be provided on the outer side of the electrode layer. The phosphorlayer may be a layer containing a known fluorescent component capable ofconverting blue light into yellow light, and is not particularlylimited. For example, it may be a combination with a phosphor that emitsyellow light, such as YAG. Accordingly, a pseudo-white light source canbe obtained because blue light that has passed through the phosphorlayer and yellow light from the phosphor are complementary. The phosphorlayer may be configured to perform both conversion of ultraviolet lightinto visible light and conversion of blue light into yellow light byincluding both a fluorescent component that converts blue into yellowand a fluorescent component that converts ultraviolet light into visiblelight.

Applications

The self-supporting gallium nitride substrate of the present inventioncan be preferably used in not only the above-described light emittingdevice but also various applications such as various electronic devices,power devices, photodetectors, and solar cell wafers.

EXAMPLES

The present invention will now be more specifically described by way ofthe following examples.

Example 1 (1) Production of c-Plane Oriented Alumina Sintered Body

As a raw material, a plate-shaped alumina powder (manufactured by KinseiMatec Co., Ltd., grade 00610) was provided. 7 parts by weight of abinder (polyvinyl butyral: lot number BM-2, manufactured by SekisuiChemical Co., Ltd.), 3.5 parts by weight of a plasticizer (DOP:di(2-ethylhexyl) phthalate, manufactured by Kurogane Kasei Co., Ltd.), 2parts by weight of a dispersing agent (Rheodol SP-O30, manufactured byKao Corporation), and a dispersion medium (2-ethylhexanol) were mixedwith 100 parts by weight of the plate-shaped alumina particles. Theamount of the dispersion medium was adjusted so that the slurryviscosity was 20000 cP. The slurry prepared as above was formed into asheet on a PET film by a doctor blade method so as to have a drythickness of 20 μm. The resulting tape was cut into circles having adiameter of 50.8 mm (2 inches), then 150 pieces were stacked and placedon an Al plate having a thickness of 10 mm, and then vacuum packing wasperformed. This vacuum pack was subjected to isostatic pressing in hotwater at 85° C. under a pressure of 100 kgf/cm², and a disc-shaped greenbody was obtained.

The resulting green body was placed in a degreasing furnace anddegreased under conditions of 600° C. and 10 hours. The resultingdegreased body was fired in a hot pressing under conditions of 1600° C.,4 hours, and a surface pressure of 200 kgf/cm² in nitrogen using agraphite mold. The resulting sintered body was re-fired under conditionsof 1700° C., 2 hours, and a gas pressure of 1500 kgf/cm² in argon by hotisostatic pressing (HIP).

The sintered body obtained in this way was fixed to a ceramic surfaceplate and ground to #2000 using grinding wheel to flatten the platesurface. Next, the plate surface was smoothed by lapping using diamondabrasive grains to obtain an oriented alumina sintered body having adiameter of 50.8 mm (2 inches) and a thickness of 1 mm as an orientedalumina substrate. Flatness was improved by reducing the size ofabrasive grains from 3 μm to 0.5 μm in a stepwise manner. The averageroughness Ra after processing was 1 nm.

(2) Evaluation of Oriented Alumina Substrate (Evaluation of Degree ofOrientation)

To check the degree of orientation of the resulting oriented aluminasubstrate, the degree of orientation at the c-plane, which is themeasurement-target crystal plane in this experimental example, wasmeasured by XRD. An XRD profile obtained when irradiating the platesurface of the oriented alumina substrate with X rays was measuredwithin the range of 2θ=20−70° using an XRD apparatus (RINT-TTR IIImanufactured by Rigaku Corporation). The degree of c-plane orientationwas calculated according to the following formulae. As a result, thevalue of the degree of c-plane orientation in this experimental examplewas 97%.

$\begin{matrix}{{{{Degree}\mspace{14mu} {of}\mspace{14mu} c\text{-}{Plane}\mspace{14mu} {{Orientation}\mspace{11mu}\lbrack\%\rbrack}} = {\frac{p - p_{0}}{1 - p_{0}} \times 100}}{p_{0} = \frac{I_{0}(006)}{\sum{I_{0}({hkl})}}}{p = \frac{I_{s}(006)}{\sum{I_{s}({hkl})}}}} & \lbrack {{Mathematical}\mspace{14mu} {Formula}\mspace{14mu} 2} \rbrack\end{matrix}$

where I₀(hkl) and I_(S)(hkl) are the diffraction intensities (integralvalues) from the (hkl) planes in ICDD No. 461212 and a sample,respectively.

(Grain Diameter Evaluation of Sintered Body Grains)

Concerning the sintered body grains of the oriented alumina substrate,the average grain diameter at the plate surface was measured by thefollowing method. The plate surface of the resulting oriented aluminasubstrate was polished and subjected to thermal etching at 1550° C. for45 minutes, and then an image was taken with a scanning electronmicroscope. The visual field range was determined in a way such thatwhen straight lines were diagonally drawn on the obtained image, eachstraight line crossed 10 to 30 grains. The average grain diameter at theplate surface was determined by diagonally drawing two straight lines onthe obtained image, taking the average of the line segment lengthsinside all grains crossed by the straight lines, and multiplying theaverage by 1.5. As a result, the average grain diameter at the platesurface was 100 μm.

(3) Production of Ge-Doped Self-Supporting Gallium Nitride Substrate(3a) Film Formation of Seed Crystal Layer

Next, a seed crystal layer was formed on the processed oriented aluminasubstrate using MOCVD. Specifically, a 40 nm thick low-temperature GaNlayer was deposited at 530° C., and then a GaN film having a thicknessof 3 μm was laminated at 1050° C. to obtain a seed crystal substrate.

(3b) Film Formation of Ge-Doped GaN Layer by Na Flux Method

The seed crystal substrate produced in the above process was placed inthe bottom of a cylindrical, flat-bottomed alumina crucible having aninner diameter of 80 mm and a height of 45 mm, and then the crucible wasfilled with a melt composition in a glovebox. The composition of themelt composition is as follows.

-   -   Metal Ga: 60 g    -   Metal Na: 60 g    -   Germanium tetrachloride: 1.85 g

This alumina crucible was put in a vessel made of a refractory metal andsealed, and then placed on a rotatable rack of a crystal growth furnace.After the temperature and the pressure were increased to 870° C. and 4.0MPa in a nitrogen atmosphere, the solution was maintained for 50 hourswhile being rotated, and gallium nitride crystals were allowed to growwhile stirring. After the end of crystal growth, the growth vessel wascooled slowly back to room temperature for 3 hours, and then the growthvessel was taken out from the crystal growth furnace. The meltcomposition remaining in the crucible was removed using ethanol, and asample in which gallium nitride crystals grew was recovered. In theresulting sample, Ge-doped gallium nitride crystals grew on the entiresurface of the 50.8 mm (2 inches) seed crystal substrate, and thecrystal thickness was about 0.5 mm. No cracks were observed.

The oriented alumina substrate portion of the sample obtained in thisway was removed by grinding with grinding wheel to obtain Ge-dopedgallium nitride single body. The plate surface of this Ge-doped galliumnitride crystal was ground with #600 and #2000 grinding wheels toflatten the plate surface, and then the plate surface was smoothed bylapping using diamond abrasive grains to obtain a self-supportingGe-doped gallium nitride substrate having a thickness of about 300 μm.In the smoothing processing, flatness was improved by reducing the sizeof abrasive grains from 3 μm to 0.1 μm in a stepwise manner. The averageroughness Ra of the surface of the self-supporting gallium nitridesubstrate after processing was 0.2 nm.

Although an n-type semiconductor was produced by germanium doping inthis example, doping may be performed using a different element ordoping may not be performed depending on the application and thestructure.

(Evaluation of Volume Resistivity)

The in-plane volume resistivity of the self-supporting gallium nitridesubstrate was measured using a Hall effect analyzer. As a result, thevolume resistivity was 1×10⁻² Ω·cm.

(Evaluation of Cross-Sectional Average Diameter of Self-SupportingGallium Nitride Substrate)

In order to measure the cross-sectional average diameter of GaN singlecrystal grains at the outermost surface of the self-supporting galliumnitride substrate, an image of the top surface of the self-supportingsubstrate was taken with a scanning electron microscope. The visualfield range was determined in a way such that when straight lines werediagonally drawn on the obtained image, the straight lines crossed 10 to30 columnar structures. The cross-sectional average grain diameter ofGaN single crystal grains at the outermost surface of theself-supporting gallium nitride substrate was determined by diagonallydrawing two straight lines on the obtained image, taking the average ofthe line segment lengths inside all grains crossed by the straightlines, and multiplying the average by 1.5. As a result, thecross-sectional average diameter was about 100 μm. In this example, itwas possible to clearly determine the interface on the scanningmicroscope image of the top surface, but the above evaluation may becarried out after performing processing to emphasize the interface bythermal etching or chemical etching.

(4) Production of Light Emitting Device Including Ge-DopedSelf-Supporting Gallium Nitride Substrate (4a) Film Formation of LightEmitting Functional Layer by MOCVD

Using MOCVD, a 1 μm thick n-GaN layer doped to give a Si atomconcentration of 5×10¹⁸/cm³ was deposited at 1050° C. as an n-type layeron the self-supporting gallium nitride substrate. Next, a multiplequantum well layer was deposited at 750° C. as a light emitting layer.Specifically, five 2.5 nm thick InGaN well layers and six 10 nm thickGaN barrier layers were alternately stacked. Next, a 200 nm thick p-GaNdoped to give a Mg atom concentration of 1×10¹⁹/cm³ was deposited at950° C. as a p-type layer. Thereafter, the sample was taken out from theMOCVD apparatus, and 800° C. heat treatment was performed for 10 minutesin a nitrogen atmosphere as activation treatment of Mg ions of thep-type layer. In order to measure the cross-sectional average diameterof single crystal grains at the outermost surface of the light emittingfunctional layer, an image of the top surface of the light emittingfunctional layer was taken with a scanning electron microscope. Thevisual field range was determined in a way such that when straight lineswere diagonally drawn on the obtained image, the straight lines crossed10 to 30 columnar structures. The cross-sectional average grain diameterof single crystal grains at the outermost surface of the light emittingfunctional layer was determined by diagonally drawing two straight lineson the obtained image, taking the average of the line segment lengthsinside all grains crossed by the straight lines, and multiplying theaverage by 1.5. As a result, the cross-sectional average diameter wasabout 100 μm.

(4b) Production of Light Emitting Device

Using a photolithography process and a vacuum deposition method,Ti/Al/Ni/Au films as a cathode electrode were patterned on the surfaceon the side opposite to the n-GaN layer and the p-GaN layer of theself-supporting gallium nitride substrate in a thickness of 15 nm, 70nm, 12 nm, and 60 nm, respectively. Thereafter, to improve ohmic contactcharacteristics, 700° C. heat treatment was performed in a nitrogenatmosphere for 30 seconds. Furthermore, using a photolithography processand a vacuum deposition method, Ni/Au films were patterned as atranslucent anode electrode on the p-type layer in a thickness of 6 nmand 12 nm, respectively. Thereafter, to improve ohmic contactcharacteristics, 500° C. heat treatment was performed in a nitrogenatmosphere for 30 seconds. Furthermore, using a photolithography processand a vacuum deposition method, Ni/Au films that served as an anodeelectrode pad were patterned in a thickness of 5 nm and 60 nm,respectively, on a partial area of the top surface of the aforementionedNi/Au films as a translucent anode electrode. The wafer obtained in thisway was cut into a chip and, further, furnished with a lead frame toobtain a light emitting device having a vertical structure.

(4c) Evaluation of Light Emitting Device

When electricity was applied across the cathode electrode and the anodeelectrode and I-V measurement was performed, rectifying characteristicswere confirmed. Moreover, when an electric current was applied in theforward direction, emission of light having a wavelength of 450 nm wasconfirmed.

Example 2 (1) Production of Mg-Doped Self-Supporting Gallium NitrideSubstrate

A seed crystal substrate in which a GaN film having a thickness of 3 μmwas stacked on an oriented alumina substrate was produced by a methodsimilar to (1) to (3) of Example 1. A Mg-doped GaN film was formed onthis seed crystal substrate as in (3b) of Example 1 except that thecomposition of the melt composition was as follows.

-   -   Metal Ga: 60 g    -   Metal Na: 60 g    -   Metal Mg: 0.02 g

In the resulting sample, Mg-doped gallium nitride crystals grew on theentire surface of the 50.8 mm (2 inches) seed crystal substrate, and thecrystal thickness was about 0.5 mm. No cracks were observed. Moreover,the Mg concentration in the resulting gallium nitride was 4×10¹⁹/cm³,and the Hall concentration measured using a Hall effect analyzer was1×10¹⁸/cm³. The oriented alumina substrate portion of the sampleobtained in this way was removed by grinding with grinding wheel toobtain a Mg-doped gallium nitride single body. The plate surface of thisMg-doped gallium nitride crystal was ground with #600 and #2000 grindingwheels to flatten the plate surface, and then the plate surface wassmoothed by lapping using diamond abrasive grains to obtain a Mg-dopedself-supporting gallium nitride substrate having a thickness of about150 μm. In the smoothing processing, flatness was improved by reducingthe size of abrasive grains from 3 μm to 0.1 μm in a stepwise manner.The average roughness Ra of the surface of the Mg-doped self-supportinggallium nitride substrate after processing was 0.2 nm. Measurement ofthe cross-sectional average diameter of the Mg-doped self-supportinggallium nitride substrate by a method similar to (3b) of Example 1resulted in a cross-sectional average diameter of about 100 μm.

(2) Production of Light Emitting Device Including Mg-DopedSelf-Supporting Gallium Nitride Substrate

(2a) Film Formation of p-Type Layer by MOCVD

Using MOCVD, a 200 nm thick p-GaN doped to give a Mg atom concentrationof 1×10¹⁹/cm³ was deposited at 950° C. as a p-type layer on a substrate.Thereafter, the sample was taken out from the MOCVD apparatus, and 800°C. heat treatment was performed for 10 minutes in a nitrogen atmosphereas activation treatment of Mg ions of the p-type layer.

(2b) Film Formation of n-Type Layer by RS-MBE and Hydrothermal Method(2b-1) Film Formation of Seed Crystal Layer by RS-MBE

Using an RS-MBE (radical source molecular beam epitaxy) apparatus, zinc(Zn) and aluminum (Al), which are metal materials, were irradiated in aKnudsen cell and supplied onto the p-type layer. Oxygen (O), which is agaseous material, was supplied as an oxygen radical with an RF radicalgenerator in which O₂ gas was used as a raw material. As for the purityof various raw materials that were used, the purity for Zn was 7N whilethat for O₂ was 6N. The substrate was heated to 700° C. using aresistance heater, and a film of an Al-doped n-ZnO seed crystal layerhaving a thickness of 20 nm was formed while controlling the flux ofvarious gas sources so that the Al concentration was 2×10¹⁸/cm³ and theratio of the Zn atom concentration to the 0 atom concentration was 1 to1 in the film.

(2b-2) Film Formation of n-Type Layer by Hydrothermal Method

Zinc nitrate was dissolved in pure water so as to be 0.1 M to givesolution A. Next, 1 M aqueous ammonia was provided as solution B. Next,aluminium sulfate was dissolved in pure water so as to be 0.1 M to givesolution C. These solutions were mixed and stirred so that the volumeratio was solution A:solution B:solution C=1:1:0.01 to obtain an aqueousgrowth solution.

The self-supporting gallium nitride substrate on which a film of a seedcrystal layer had been formed was placed upright in 1 liter of theaqueous growth solution. Next, a waterproofed ceramic heater and amagnetic stirrer were placed in the aqueous solution, the aqueoussolution was placed in an autoclave to perform hydrothermal treatment at270° C. for 3 hours, and a ZnO layer was precipitated on the seedcrystal layer. The self-supporting gallium nitride substrate on which aZnO layer had been deposited was washed with pure water, and thenannealing treatment was performed at 500° C. in air to form an Al-dopedn-ZnO layer having a thickness of about 3 μm. Neither pores nor crackswere detected in the sample, and electroconductivity of the ZnO layerwas confirmed by using a tester. As a result of evaluating thecross-sectional average diameter of the light emitting functional layerusing a method similar to (4a) of Example 1, the cross-sectional averagediameter of single crystal grains at the outermost surface of the lightemitting functional layer was about 100 μm.

(2c) Production of Light Emitting Device

Using a photolithography process and a vacuum deposition method,Ti/Al/Ni/Au films as a cathode electrode were patterned on the n-typelayer in a thickness of 15 nm, 70 nm, 12 nm, and 60 nm, respectively.The cathode electrode was patterned into a shape having an opening sothat light can be extracted from a portion where an electrode was notformed. Thereafter, to improve ohmic contact characteristics, 700° C.heat treatment was performed in a nitrogen atmosphere for 30 seconds.Moreover, using a photolithography process and a vacuum depositionmethod, Ni/Au films as an anode electrode were patterned on the surfaceon the side opposite to the p-GaN layer and the n-ZnO layer of theself-supporting gallium nitride substrate in a thickness of 50 nm and100 nm, respectively. Thereafter, to improve ohmic contactcharacteristics, 500° C. heat treatment was performed in a nitrogenatmosphere for 30 seconds. The wafer obtained in this way was cut into achip and, further, furnished with a lead frame to obtain a lightemitting device having a vertical structure.

(2d) Evaluation of Light Emitting Device

When electricity was applied across the cathode electrode and the anodeelectrode and I-V measurement was performed, rectifying characteristicswere confirmed. Moreover, when an electric current was applied in theforward direction, emission of light having a wavelength of about 380 nmwas confirmed.

Example 3 (1) Production of Light Emitting Device Including Mg-DopedSelf-Supporting Gallium Nitride Substrate (1a) Film Formation of ActiveLayer by RS-MBE

A Mg-doped self-supporting gallium nitride substrate was produced by amethod similar to (1) and (2a) of Example 2, and 200 nm thick p-GaN wasdeposited on the substrate as a p-type layer. Next, using an RS-MBE(radical source molecular beam epitaxy) apparatus, zinc (Zn) and cadmium(Cd), which are metal materials, were irradiated in a Knudsen cell andsupplied onto the p-type layer. Oxygen (O), which is a gaseous material,was supplied as an oxygen radical with an RF radical generator in whichO₂ gas was used as a raw material. As for the purity of various rawmaterials that were used, the purity for Zn and Cd was 7N while that forO₂ was 6N. The substrate was heated to 700° C. using a resistanceheater, and a film of an active layer having a thickness of 1.5 nm wasformed while controlling the flux of various gas sources so as toprovide a Cd_(0.2)Zn_(0.8)O layer.

(1b) Film Formation of n-Type Layer by Sputtering

Next, a 500 nm thick film of an n-type ZnO layer was formed on theactive layer using RF magnetron sputtering. For film formation, a ZnOtarget to which 2 parts by weight of Al had been added was used, andfilm formation conditions included a pure Ar atmosphere, a pressure of0.5 Pa, an applied power of 150 W, and a film formation time of 5minutes. As a result of evaluating the cross-sectional average diameterof the light emitting functional layer using a method similar to (4a) ofExample 1, the average grain diameter at the plate surface of the lightemitting functional layer was about 100 μm.

(1c) Production of Light Emitting Device

Using a photolithography process and a vacuum deposition method,Ti/Al/Ni/Au films as a cathode electrode were patterned on the n-typelayer in a thickness of 15 nm, 70 nm, 12 nm, and 60 nm, respectively.The cathode electrode was patterned into a shape having an opening sothat light can be extracted from a portion where an electrode was notformed. Thereafter, to improve ohmic contact characteristics, 700° C.heat treatment was performed in a nitrogen atmosphere for 30 seconds.Moreover, using a photolithography process and a vacuum depositionmethod, Ni/Au films as an anode electrode were patterned on the surfaceon the side opposite to the p-GaN layer and the n-ZnO layer of theself-supporting gallium nitride substrate in a thickness of 5 nm and 100nm, respectively. Thereafter, to improve ohmic contact characteristics,500° C. heat treatment was performed in a nitrogen atmosphere for 30seconds. The wafer obtained in this way was cut into a chip and,further, furnished with a lead frame to obtain a light emitting devicehaving a vertical structure.

(1d) Evaluation of Light Emitting Device

When electricity was applied across the cathode electrode and the anodeelectrode and I-V measurement was performed, rectifying characteristicswere confirmed. Moreover, when an electric current was applied in theforward direction, emission of light having a wavelength of about 400 nmwas confirmed.

Example 4 (1) Production of c-Plane Oriented Alumina Sintered Body

A disc-shaped green body was obtained as in (1) of Example 1. Theresulting green body was placed in a degreasing furnace and degreasedunder conditions of 600° C. and 10 hours. The resulting degreased bodywas fired with a hot pressing under conditions of 1700° C., 4 hours, anda surface pressure of 200 kgf/cm² in nitrogen using a graphite mold.

The sintered body obtained in this way was fixed to a ceramic surfaceplate and ground to #2000 using grinding wheel to flatten the platesurface. Next, the plate surface was smoothed by lapping using diamondabrasive grains to obtain an oriented alumina sintered body having adiameter of 50.8 mm (2 inches) and a thickness of 1 mm as an orientedalumina substrate. Flatness was improved by reducing the size ofabrasive grains from 3 μm to 0.5 μm in a stepwise manner. The averageroughness Ra after processing was 4 nm. Evaluation of the degree ofc-plane orientation and the average grain diameter at the plate surfaceby methods similar to Example 1 resulted in a degree of c-planeorientation of 99% and an average grain diameter of 18 μm.

(2) Production of Ge-Doped Self-Supporting Gallium Nitride Substrate

A seed crystal substrate in which a GaN film having a thickness of 3 μmwas stacked on the oriented alumina substrate was produced as in (3a) ofExample 1. A Ge-doped GaN film was formed on this seed crystal substrateas in (3b) of Example 1 except that the retention time was 20 hours. Inthe resulting sample, Ge-doped gallium nitride crystals grew on theentire surface of the 50.8 mm (2 inches) seed crystal substrate, and thecrystal thickness was about 0.2 mm. No cracks were observed.

The plate surface (top surface) of Ge-doped gallium nitride crystals ofthe sample thus obtained was ground and flattened with #600 and #2000grinding wheels until the thickness of the gallium nitride crystalsbecame about 50 μm, and then the plate surface was smoothed by lappingusing diamond abrasive grains. Next, the sample was cut to expose theplane perpendicular to the plate surface, the plane was polished using aCP polisher (manufactured by JEOL Ltd., IB-09010CP), and then inversepole figure mapping of the cross section of the gallium nitride crystalswas performed with an electron-beam backscattering diffractometer (EBSD)(manufactured by TSL Solutions). FIG. 2 shows an inverse pole figuremap. FIG. 3 shows an inverse pole figure map measured at the platesurface (top surface) of the gallium nitride crystals, and FIG. 4 showsa crystal grain map image in which the interface between the orientedalumina substrate and the gallium nitride crystals is enlarged. It canbe understood from FIG. 2 that the gallium nitride crystals have alarger grain diameter on the top surface side (the side opposite to theoriented alumina substrate) than the oriented alumina substrate side,and the shape of the gallium nitride crystals, being trapezoidal,triangular, or the like on the cross-sectional image, is not completelycolumnar. Moreover, it can be understood that, as the film thickens,there are grains whose grain diameter increases and that grow to the topsurface, and grains that do not grow to the top surface. FIG. 3 showsthat the c-plane of each grain constituting the gallium nitride crystalsis mostly oriented in the normal direction. Moreover, it can beunderstood from FIG. 4 that gallium nitride crystal grains grow, withcrystal grains constituting the oriented alumina substrate that servesas a base being a starting point. Although the cause of such a growthbehavior that the grain diameter increases as the film thickens is notclear, it is considered that, as conceptually shown in FIG. 5, growthhas progressed so that fast-growing grains cover slow-growing grains.Therefore, among the gallium nitride grains constituting the galliumnitride crystals, grains exposed on the top surface side connect to thebottom surface without intervention of a grain boundary, but grainsexposed on the bottom surface side include grains which ceased to growhalfway.

Next, the oriented alumina substrate portion of the sample was removedby grinding with grinding wheel to obtain a Ge-doped gallium nitridesingle body. The bottom surface (the surface on the side that had beenin contact with the oriented alumina substrate) of the Ge-doped galliumnitride crystals was subjected to lapping with diamond abrasive grainsto obtain a self-supporting gallium nitride substrate, the plate topsurface (the side opposite to the side that had been in contact with theoriented alumina substrate) and the bottom surface (the surface on theside that had been in contact with the oriented alumina substrate) ofwhich were smoothed. The average roughness Ra of the top surface and thebottom surface of the self-supporting gallium nitride substrate afterprocessing was 0.2 nm.

The volume resistivity was measured by a method similar to (3) ofExample 1, and the volume resistivity was 1×10⁻² Ω·cm. Moreover, as aresult of measuring the cross-sectional average diameter of GaN singlecrystal grains at the top surface and the bottom surface of theself-supporting gallium nitride substrate using a method similar to (3)of Example 1, the cross-sectional average diameter at the top surfacewas about 50 μm, and the cross-sectional average diameter at the bottomsurface was about 18 μm. In this way, the cross-sectional averagediameter was larger at the top surface than the bottom surface, andD_(T)/D_(B), which is the ratio of the cross-sectional average diameterD_(T) at the substrate top surface to the cross-sectional averagediameter D_(B) of the substrate bottom surface, was about 2.8. Moreover,the aspect ratio of GaN single crystal grains calculated as the ratio ofthe thickness of GaN crystals to the cross-sectional average diameter atthe top surface was about 1.0.

(3) Production of Light Emitting Device Including Ge-DopedSelf-Supporting Gallium Nitride Substrate

A light emitting functional layer was produced on the self-supportinggallium nitride substrate as in (4a) of Example 1, and measurement ofthe cross-sectional average diameter of single crystal grains at theoutermost surface resulted in a cross-sectional average diameter ofabout 50 μm. Moreover, as a result of producing a vertical lightemitting device as in (4b) of Example 1, rectifying characteristics wereconfirmed by I-V measurement between the cathode electrode and the anodeelectrode, and application of electricity in the forward directionconfirmed emission of light having a wavelength of 450 nm.

For reference, a self-supporting substrate having a thickness of 20 μmwas also provided by grinding the top surface side of a self-supportinggallium nitride substrate produced as in (1) and (2) described above. Atthis time, the cross-sectional average diameter of single crystal grainsat the outermost surface was about 35 μm; D_(T)/D_(B), which is theratio of the cross-sectional average diameter D_(T) at the substrate topsurface to the cross-sectional average diameter D_(B) of the substratebottom surface, was 1.9; and the aspect ratio was about 0.6. When alight emitting functional layer as above was produced on theself-supporting oriented GaN crystals to form a vertical light emittingdevice and then an electric current was applied in the forwarddirection, rectifying characteristics and an emission of light having awavelength of 450 nm were both confirmed and also the luminance ofemitted light was somewhat high, but the luminance of emitted light waslower than that of the aforementioned device.

Example 5 (1) Production of c-Plane Oriented Alumina Sintered Body

A plate-shaped alumina powder (manufactured by Kinsei Matec Co., Ltd.,grade 02025), a fine alumina powder (manufactured by Taimei ChemicalsCo., Ltd., grade TM-DAR), and a magnesium oxide powder (manufactured byUbe Material Industries, Ltd., grade 500 A) were provided as rawmaterials, and 5 parts by weight of the plate-shaped alumina powder, 95weight of the fine alumina powder, and 0.025 weight of the magnesiumoxide powder were mixed to obtain an alumina raw material. Next, 8 partsby weight of a binder (polyvinyl butyral: product name BM-2,manufactured by Sekisui Chemical Co., Ltd.), 4 parts by weight of aplasticizer (DOP: di(2-ethylhexyl) phthalate, manufactured by KuroganeKasei Co., Ltd.), 2 parts by weight of a dispersing agent (RheodolSP-030, manufactured by Kao Corporation), and a dispersion medium (amixture of xylene and 1-butanol in a weight ratio of 1:1) were mixedwith 100 parts by weight of the alumina raw material. The amount of thedispersion medium was adjusted so that the slurry viscosity was 20000cP. The slurry prepared as above was formed into a sheet on a PET filmby a doctor blade method so as to have a thickness after drying of 100μm. The resulting tape was cut into circles having a diameter of 50.8 mm(2 inches), then 30 pieces were stacked and placed on an Al plate havinga thickness of 10 mm, and then vacuum packing was performed. This vacuumpack was subjected to isostatic pressing in hot water at 85° C. under apressure of 100 kgf/cm², and a disc-shaped green body was obtained.

The resulting green body was placed in a degreasing furnace anddegreased under conditions of 600° C. and 10 hours. The resultingdegreased body was fired with a hot pressing under conditions of 1800°C., 4 hours, and a surface pressure of 200 kgf/cm² in nitrogen using agraphite mold.

The sintered body obtained in this way was fixed to a ceramic surfaceplate and ground to #2000 using grinding wheel to flatten the platesurface. Next, the plate surface was smoothed by lapping using diamondabrasive grains to obtain an oriented alumina sintered body having adiameter of 50.8 mm (2 inches) and a thickness of 1 mm as an orientedalumina substrate. Flatness was improved by reducing the size ofabrasive grains from 3 μm to 0.5 μm in a stepwise manner. The averageroughness Ra after processing was 4 nm. Evaluation of the degree ofc-plane orientation and the average grain diameter at the plate surfaceby methods similar to Example 1 resulted in a degree of c-planeorientation of 96% and an average grain diameter of about 20 μm.

(2) Production of Ge-Doped Self-Supporting Gallium Nitride Substrate

A seed crystal substrate in which a GaN film having a thickness of 3 μmwas stacked on the oriented alumina substrate was produced as in (3a) ofExample 1. A Ge-doped GaN film was formed on this seed crystal substrateas in (3b) of Example 1 except that the retention time was 30 hours. Inthe resulting sample, Ge-doped gallium nitride crystals grew on theentire surface of the 50.8 mm (2 inches) seed crystal substrate, and thecrystal thickness was about 0.3 mm. No cracks were observed.

The plate surface (top surface) of Ge-doped gallium nitride crystals ofthe sample thus obtained was ground and flattened with #600 and #2000grinding wheels until the thickness of the gallium nitride crystalsbecame about 180 μm, and then the plate surface was smoothed by lappingusing diamond abrasive grains. Next, the sample was cut to expose theplane perpendicular to the plate surface, the plane was polished using aCP polisher (manufactured by JEOL Ltd., IB-09010CP), and then inversepole figure mapping of the cross section of the gallium nitride crystalswas performed with an electron-beam backscattering diffractometer (EBSD)(manufactured by TSL Solutions). FIG. 6 shows an inverse pole figuremap. It can be understood from FIG. 6 that the gallium nitride crystalshave a larger grain diameter on the top surface side (the side oppositeto the oriented alumina substrate) than the oriented alumina substrateside, and the shape of the gallium nitride crystals, being trapezoidal,triangular, or the like on the cross-sectional image, is not completelycolumnar. Moreover, it can be understood that, as the film thickens,there are grains whose grain diameter increases and that grow to the topsurface, and grains that do not grow to the top surface. Although thecause of such a growth behavior is not clear, it is considered that, asshown in FIG. 5, such a behavior is a result of growth that progressedin such a manner that fast-growing grains covered slow-growing grains.Therefore, among the gallium nitride grains constituting the galliumnitride crystals, grains exposed on the top surface side connect to thebottom surface without intervention of a grain boundary, but grainsexposed on the bottom surface side include grains which ceased to growhalfway.

Next, the oriented alumina substrate portion of the sample was removedby grinding with grinding wheel to obtain a Ge-doped gallium nitridesingle body. The bottom surface (the surface on the side that had beenin contact with the oriented alumina substrate) of the Ge-doped galliumnitride crystals was subjected to lapping with diamond abrasive grainsto obtain a self-supporting gallium nitride substrate having a thicknessof about 180 μm, the plate top surface (the side opposite to the sidethat had been in contact with the oriented alumina substrate) and thebottom surface (the surface on the side that had been in contact withthe oriented alumina substrate) of which were smoothed. The averageroughness Ra of the top surface and the bottom surface of theself-supporting gallium nitride substrate after processing was 0.2 nm.

The volume resistivity was measured by a method similar to (3) ofExample 1, and the volume resistivity was 1×10⁻² Ω·cm. Moreover, as aresult of measuring the cross-sectional average diameter of GaN singlecrystal grains at the top surface and the bottom surface of theself-supporting gallium nitride substrate using a method similar to (3)of Example 1, the cross-sectional average diameter at the top surfacewas about 150 μm, and the cross-sectional average diameter at the bottomsurface was about 20 μm. In this way, the cross-sectional averagediameter was larger at the top surface than the bottom surface, andD_(T)/D_(B), which is the ratio of the cross-sectional average diameterD_(T) at the substrate top surface to the cross-sectional averagediameter D_(B) of the substrate bottom surface, was about 7.5. Moreover,the aspect ratio of GaN single crystal grains calculated as the ratio ofthe thickness of GaN crystals to the cross-sectional average diameter atthe top surface was about 1.2.

(3) Production of Light Emitting Device Including Ge-DopedSelf-Supporting Gallium Nitride Substrate

A light emitting functional layer was produced on the self-supportinggallium nitride substrate as in (4a) of Example 1, and measurement ofthe cross-sectional average diameter of single crystal grains at theoutermost surface resulted in a cross-sectional average diameter ofabout 150 μm. Moreover, as a result of producing a vertical lightemitting device as in (4b) of Example 1, rectifying characteristics wereconfirmed by I-V measurement between the cathode electrode and the anodeelectrode, and application of electricity in the forward directionconfirmed emission of light having a wavelength of 450 nm.

For reference, a self-supporting substrate having a thickness of 50 μmand a self-supporting substrate having a thickness of 20 μm wereprovided by grinding the top surface side of a self-supporting galliumnitride substrate produced as in (1) and (2) above. The cross-sectionalaverage diameter of single crystal grains at the outermost surface ofthe self-supporting substrate having a thickness of 50 μm was about 63μm; D_(T)/D_(B), which is the ratio of the cross-sectional averagediameter D_(T) at the substrate top surface to the cross-sectionalaverage diameter D_(B) of the substrate bottom surface, was 3.2; and theaspect ratio was about 0.8. When a light emitting functional layer asabove was produced on the self-supporting oriented GaN crystals to forma vertical light emitting device and then an electric current wasapplied in the forward direction, rectifying characteristics and anemission of light having a wavelength of 450 nm were both confirmed andalso the luminance of emitted light was somewhat high, but the luminanceof emitted light was lower than that of the aforementioned device. Thecross-sectional average diameter of single crystal grains at theoutermost surface of the self-supporting substrate having a thickness of20 μm was about 39 μm; D_(T)/D_(B), which is the ratio of thecross-sectional average diameter D_(T) at the substrate top surface tothe cross-sectional average diameter D_(B) of the substrate bottomsurface, was 2.0; and the aspect ratio was about 0.5. When a lightemitting functional layer as above was produced on the self-supportingoriented GaN crystals to form a vertical light emitting device and thenan electric current was applied in the forward direction, rectifyingcharacteristics and an emission of light having a wavelength of 450 nmwere both confirmed and also the luminance of emitted light was somewhathigh, but the luminance of emitted light was further lower than those ofthe two aforementioned devices.

Example 6 (1) Production of Ge-Doped Self-Supporting Gallium NitrideSubstrate

A c-plane oriented alumina substrate was produced as in Example 5, and aGaN film having a thickness of 3 μm was stacked to produce a seedcrystal substrate. A Ge-doped GaN film was formed on this seed crystalsubstrate as in (3b) of Example 1 except that the retention time was 40hours. In the resulting sample, Ge-doped gallium nitride crystals grewon the entire surface of the 50.8 mm (2 inches) seed crystal substrate,and the crystal thickness was about 0.4 mm. No cracks were observed.

The plate surface (top surface) of Ge-doped gallium nitride crystals ofthe sample thus obtained was ground and flattened with #600 and #2000grinding wheels until the thickness of the gallium nitride crystalsbecame about 260 μm, and then the plate surface was smoothed by lappingusing diamond abrasive grains. Next, inverse pole figure mapping of thecross section of the gallium nitride crystals was performed using amethod similar to Examples 4 and 5, and it was found that the galliumnitride crystals had a larger grain diameter on the top surface side(the side opposite to the oriented alumina substrate) than the orientedalumina substrate side, and the shape of the gallium nitride crystals,being trapezoidal, triangular, or the like on the cross-sectional image,was not completely columnar. Moreover, it was found that, as the filmthickened, there were grains whose grain diameter increased and thatgrew to the top surface, and grains that did not grow to the topsurface. Although the cause of such a growth behavior is not clear, itis considered that, as shown in FIG. 5, such a behavior is a result ofgrowth that progressed in such a manner that fast-growing grains coveredslow-growing grains. Therefore, among the gallium nitride grainsconstituting the gallium nitride crystals, grains exposed on the topsurface side connect to the bottom surface without intervention of agrain boundary, but grains exposed on the bottom surface side includegrains which ceased to grow halfway.

Next, the oriented alumina substrate portion of the sample was removedby grinding with grinding wheel to obtain a Ge-doped gallium nitridesingle body. The bottom surface (the surface on the side that had beenin contact with the oriented alumina substrate) of the Ge-doped galliumnitride crystals was subjected to lapping with diamond abrasive grainsto obtain a self-supporting gallium nitride substrate having a thicknessof about 260 μm, the plate top surface (the side opposite to the sidethat had been in contact with the oriented alumina substrate) and thebottom surface (the surface on the side that had been in contact withthe oriented alumina substrate) of which were smoothed. The averageroughness Ra of the top surface and the bottom surface of theself-supporting gallium nitride substrate after processing was 0.2 nm.

The volume resistivity was measured by a method similar to (3) ofExample 1, and the volume resistivity was 1×10⁻² Ω·cm. Moreover, as aresult of measuring the cross-sectional average diameter of GaN singlecrystal grains at the top surface and the bottom surface of theself-supporting gallium nitride substrate using a method similar to (3)of Example 1, the cross-sectional average diameter at the top surfacewas about 220 μm, and the cross-sectional average diameter at the bottomsurface was about 20 μm. In this way, the cross-sectional averagediameter was larger at the top surface than the bottom surface, andD_(T)/D_(B), which is the ratio of the cross-sectional average diameterD_(T) at the substrate top surface to the cross-sectional averagediameter D_(B) of the substrate bottom surface, was about 11.0.Moreover, the aspect ratio of GaN single crystal grains calculated asthe ratio of the thickness of GaN crystals to the cross-sectionalaverage diameter at the top surface was about 1.2.

(2) Production of Light Emitting Device Including Ge-DopedSelf-Supporting Gallium Nitride Substrate

A light emitting functional layer was produced on the self-supportinggallium nitride substrate as in (4a) of Example 1, and measurement ofthe cross-sectional average diameter of single crystal grains at theoutermost surface resulted in a cross-sectional average diameter ofabout 220 μm. Moreover, as a result of producing a vertical lightemitting device as in (4b) of Example 1, rectifying characteristics wereconfirmed by I-V measurement between the cathode electrode and the anodeelectrode, and application of electricity in the forward directionconfirmed emission of light having a wavelength of 450 nm. It was foundthat the luminance was somewhat high but lower than that of the deviceof Example 5.

Example 7 (1) Production of c-Plane Oriented Alumina Sintered Body

A c-plane oriented alumina substrate was produced as in Example 1 exceptthat the firing temperature with a hot pressing was 1750° C. Thesintered body obtained in this way was fixed to a ceramic surface plateand ground to #2000 using grinding wheel to flatten the plate surface.Next, the plate surface was smoothed by lapping using diamond abrasivegrains to obtain an oriented alumina sintered body having a diameter of50.8 mm (2 inches) and a thickness of 1 mm as an oriented aluminasubstrate. Flatness was improved by reducing the size of abrasive grainsfrom 3 μm to 0.5 μm in a stepwise manner. The average roughness Ra afterprocessing was 4 nm. Evaluation of the degree of c-plane orientation andthe average grain diameter at the plate surface by methods similar toExample 1 resulted in a degree of c-plane orientation of 96% and anaverage grain diameter of 14 μm.

(2) Production of Ge-Doped Self-Supporting Gallium Nitride Substrate

A seed crystal substrate in which a GaN film having a thickness of 3 μmwas stacked on the oriented alumina substrate was produced as in (3a) ofExample 1. A Ge-doped GaN film was formed on this seed crystal substrateas in (3b) of Example 1 except that the retention time was 30 hours. Inthe resulting sample, Ge-doped gallium nitride crystals grew on theentire surface of the 50.8 mm (2 inches) seed crystal substrate, and thecrystal thickness was about 0.3 mm. No cracks were observed.

The plate surface (top surface) of Ge-doped gallium nitride crystals ofthe sample thus obtained was ground and flattened with #600 and #2000grinding wheels until the thickness of the gallium nitride crystalsbecame about 90 μm, and then the plate surface was smoothed by lappingusing diamond abrasive grains. Next, inverse pole figure mapping of thecross section of the gallium nitride crystals was performed using amethod similar to Examples 4 to 6, and it was found that the galliumnitride crystals had a larger grain diameter on the top surface side(the side opposite to the oriented alumina substrate) than the orientedalumina substrate side, and the shape of the gallium nitride crystals,being trapezoidal, triangular, or the like on the cross-sectional image,was not completely columnar. Moreover, it was found that, as the filmthickens, there were grains whose grain diameter increased and that grewto the top surface, and grains that did not grow to the top surface.Although the cause of such a growth behavior is not clear, it isconsidered that, as shown in FIG. 5, such a behavior is a result ofgrowth that progressed in such a manner that fast-growing grains coveredslow-growing grains. Therefore, among the gallium nitride grainsconstituting the gallium nitride crystals, grains exposed on the topsurface side connect to the bottom surface without intervention of agrain boundary, but grains exposed on the bottom surface side includegrains which ceased to grow halfway.

Next, the oriented alumina substrate portion of the sample was removedby grinding with grinding wheel to obtain a Ge-doped gallium nitridesingle body. The bottom surface (the surface on the side that had beenin contact with the oriented alumina substrate) of the Ge-doped galliumnitride crystals was subjected to lapping by diamond abrasive grains toobtain a self-supporting gallium nitride substrate having a thickness ofabout 90 μm, the plate top surface (the side opposite to the side thathad been in contact with the oriented alumina substrate) and the bottomsurface (the surface on the side that had been in contact with theoriented alumina substrate) of which were smoothed (Example 7-1). Theaverage roughness Ra of the top surface and the bottom surface of theself-supporting gallium nitride substrate after processing was 0.2 nm.

Moreover, Ge-doped gallium nitride crystals were produced as above andthe plate surface (top surface) thereof was ground using #600 and #2000grinding wheels to produce samples in which the thicknesses of thegallium nitride crystals were 70, 50, 30, and 20 μm, and the platesurfaces were smoothed by lapping using diamond abrasive grains. Next,the alumina substrate portions were removed as above, the bottomsurfaces (the surfaces on the side that had been in contact with theoriented alumina substrates) of the Ge-doped gallium nitride crystalswere subjected to lapping by diamond abrasive grains to obtainself-supporting gallium nitride substrates having thicknesses of 70, 50,30, and 20 μm, the plate top surfaces (the side opposite to the sidethat had been in contact with the oriented alumina substrates) and thebottom surfaces (the surfaces on the side that had been in contact withthe oriented alumina substrates) of which were smoothed (Example 7-2 toExample 7-5). The average roughness Ra of the top surface and the bottomsurface of each sample after processing was 0.2 nm.

The volume resistivity of each sample was measured by a method similarto (3) of Example 1, and the volume resistivity was all 1×10⁻² Ω·cm.Moreover, as a result of measuring the cross-sectional average diameterof GaN single crystal grains at the top surface and the bottom surfaceof each self-supporting gallium nitride substrate using a method similarto (3) of Example 1, the thickness of each self-supporting galliumnitride substrate, the cross-sectional average diameter at the topsurface, the cross-sectional average diameter at the bottom surface, theratio D_(T)/D_(B) of the cross-sectional average diameter D_(T) of thesubstrate top surface to the cross-sectional average diameter D_(B) ofthe substrate bottom surface, and the aspect ratio of GaN single crystalgrains calculated as the ratio of the thickness of GaN crystals to thecross-sectional average diameter at the top surface were as shown inTable 1.

TABLE 1 Average Thickness cross-sectional of self- diameter supportingat outermost gallium surface of nitride light emitting substrate D_(T)D_(B) Aspect functional layer No. (μm) (μm) (μm) D_(T)/D_(B) ratio (μm)Ex. 7-1 90 76 14 5.4 1.2 76 Ex. 7-2 70 59 14 4.2 1.2 59 Ex. 7-3 50 42 143.0 1.2 42 Ex. 7-4 30 25 14 1.8 1.2 25 Ex. 7-5 20 17 14 1.2 1.2 17

(3) Production of Light Emitting Device Including Ge-DopedSelf-Supporting Gallium Nitride Substrate

A light emitting functional layer was produced on each self-supportinggallium nitride substrate as in (4a) of Example 1. The result ofmeasuring the cross-sectional average diameter of single crystal grainsat the outermost surface is shown in Table 1. As a result of producingvertical light emitting devices as in (4b) of Example 1, rectifyingcharacteristics were confirmed by I-V measurement between the cathodeelectrode and the anode electrode in all samples, and application ofelectricity in the forward direction confirmed emission of light havinga wavelength of 450 nm. The luminances were all somewhat high and had arelationship of Example 7-1>Example 7-2>Example 7-3>Example 7-4>Example7-5.

Example 8 (1) Production of c-Plane Oriented Alumina Sintered Body

A plate-shaped alumina powder (manufactured by Kinsei Matec Co., Ltd.,grade 02025), a fine alumina powder (manufactured by Taimei ChemicalsCo., Ltd., grade TM-DAR), aluminum fluoride (manufactured by KantoChemical Co., Inc.), and a magnesium oxide powder (manufactured by UbeMaterial Industries, Ltd., grade 500 A) were provided as raw materials,and 5 parts by weight of the plate-shaped alumina powder, 95 parts byweight of the fine alumina powder, 0.05 parts by weight of the aluminumfluoride powder, and 0.025 parts by weight of the magnesium oxide powderwere mixed to obtain an alumina raw material. Next, 8 parts by weight ofa binder (polyvinyl butyral: product name BM-2, manufactured by SekisuiChemical Co., Ltd.), 4 parts by weight of a plasticizer (DOP:di(2-ethylhexyl) phthalate, manufactured by Kurogane Kasei Co., Ltd.), 2parts by weight of a dispersing agent (Rheodol SP-030, manufactured byKao Corporation), and a dispersion medium (a mixture of xylene and1-butanol in a weight ratio of 1:1) were mixed with 100 parts by weightof the alumina raw material. The amount of the dispersion medium wasadjusted so that the slurry viscosity was 20000 cP. The slurry preparedas above was formed into a sheet on a PET film by a doctor blade methodso as to have a thickness after drying of 100 μm. The resulting tape wascut into circles having a diameter of 50.8 mm (2 inches), then 30 pieceswere stacked and placed on an Al plate having a thickness of 10 mm, andthen vacuum packing was performed. This vacuum pack was subjected toisostatic pressing in hot water at 85° C. under a pressure of 100kgf/cm², and a disc-shaped green body was obtained.

The resulting green body was placed in a degreasing furnace anddegreased under conditions of 600° C. and 10 hours. The resultingdegreased body was fired with a hot pressing under conditions of 1800°C., 4 hours, and a surface pressure of 200 kgf/cm² in nitrogen using agraphite mold.

The sintered body obtained in this way was fixed to a ceramic surfaceplate and ground to #2000 using grinding wheel to flatten the platesurface. Next, the plate surface was smoothed by lapping using diamondabrasive grains to obtain an oriented alumina sintered body having adiameter of 50.8 mm (2 inches) and a thickness of 1 mm as an orientedalumina substrate. Flatness was improved by reducing the size ofabrasive grains from 3 μm to 0.5 μm in a stepwise manner. The averageroughness Ra after processing was 4 nm. Evaluation of the degree ofc-plane orientation and the average grain diameter at the plate surfaceby methods similar to Example 1 resulted in a degree of c-planeorientation of 92% and an average grain diameter of about 64 μm.

(2) Production of Ge-Doped Self-Supporting Gallium Nitride Substrate

A seed crystal substrate in which a GaN film having a thickness of 3 μmwas stacked on the oriented alumina substrate was produced as in (3a) ofExample 1. A Ge-doped GaN film was formed on this seed crystal substrateas in (3b) of Example 1 except that the retention time was 30 hours. Inthe resulting sample, Ge-doped gallium nitride crystals grew on theentire surface of the 50.8 mm (2 inches) seed crystal substrate, and thecrystal thickness was about 0.3 mm. No cracks were observed.

The plate surface (top surface) of Ge-doped gallium nitride crystals ofthe sample thus obtained were ground and flattened with #600 and #2000grinding wheels until the thickness of the gallium nitride crystalsbecame about 90 μm, and then the plate surface was smoothed by lappingusing diamond abrasive grains. Next, inverse pole figure mapping of thecross section of the gallium nitride crystals was performed using amethod similar to Examples 4 to 7, and it was found that the galliumnitride crystals had a larger grain diameter on the top surface side(the side opposite to the oriented alumina substrate) than the orientedalumina substrate side, and the shape of the gallium nitride crystals,being trapezoidal, triangular, or the like on the cross-sectional image,was not completely columnar. Moreover, it was found that, as the filmthickened, there were grains whose grain diameter increased and thatgrew to the top surface, and grains that did not grow to the topsurface. Although the cause of such a growth behavior is not clear, itis considered that, as shown in FIG. 5, such a behavior is a result ofgrowth that progressed in such a manner that fast-growing grains coveredslow-growing grains. Therefore, among the gallium nitride grainsconstituting the gallium nitride crystals, grains exposed on the topsurface side connect to the bottom surface without intervention of agrain boundary, but grains exposed on the bottom surface side includegrains which ceased to grow halfway.

Next, the oriented alumina substrate portion of the sample was removedby grinding with grinding wheel to obtain a Ge-doped gallium nitridesingle body. The bottom surface (the surface on the side that had beenin contact with the oriented alumina substrate) of the Ge-doped galliumnitride crystals was subjected to lapping with diamond abrasive grainsto obtain a self-supporting gallium nitride substrate having a thicknessof about 90 μm, the plate top surface and the bottom surface (thesurface on the side that had been in contact with the oriented aluminasubstrate) of which were smoothed. The average roughness Ra of the topsurface and the bottom surface of the self-supporting gallium nitridesubstrate after processing was 0.2 nm.

The volume resistivity was measured by a method similar to (3) ofExample 1, and the volume resistivity was 1×10⁻² Ω·cm. Moreover, as aresult of measuring the cross-sectional average diameter of GaN singlecrystal grains at the top surface and the bottom surface of theself-supporting gallium nitride substrate using a method similar to (3)of Example 1, the cross-sectional average diameter at the top surfacewas about 80 μm, and the cross-sectional average diameter at the bottomsurface was about 64 μm. In this way, the cross-sectional averagediameter was larger at the top surface than the bottom surface, andD_(T)/D_(B), which is the ratio of the cross-sectional average diameterD_(T) at the substrate top surface to the cross-sectional averagediameter D_(B) of the substrate bottom surface, was about 1.3. Moreover,the aspect ratio of GaN single crystal grains calculated as the ratio ofthe thickness of GaN crystals to the cross-sectional average diameter atthe top surface was about 1.1.

(3) Production of Light Emitting Device Including Ge-DopedSelf-Supporting Gallium Nitride Substrate

A light emitting functional layer was produced on the self-supportinggallium nitride substrate as in (4a) of Example 1, and measurement ofthe cross-sectional average diameter of single crystal grains at theoutermost surface resulted in a cross-sectional average diameter ofabout 80 μm. Moreover, as a result of producing a vertical lightemitting device as in (4b) of Example 1, rectifying characteristics wereconfirmed by I-V measurement between the cathode electrode and the anodeelectrode, and application of electricity in the forward directionconfirmed emission of light having a wavelength of 450 nm.

Example 9 (1) Production of c-Plane Oriented Alumina Sintered Body

A c-plane oriented alumina substrate was produced as in Example 8 exceptthat the amount of aluminum fluoride powder was 0.02 parts by weight.The sintered body obtained in this way was fixed to a ceramic surfaceplate and ground to #2000 using grinding wheel to flatten the platesurface. Next, the plate surface was smoothed by lapping using diamondabrasive grains to obtain an oriented alumina sintered body having adiameter of 50.8 mm (2 inches) and a thickness of 1 mm as an orientedalumina substrate. Flatness was improved by reducing the size ofabrasive grains from 3 μm to 0.5 μm in a stepwise manner. The averageroughness Ra after processing was 4 nm. Evaluation of the degree ofc-plane orientation and the average grain diameter at the plate surfaceby methods similar to Example 1 resulted in a degree of c-planeorientation of 94% and an average grain diameter of 41 μm.

(2) Production of Ge-Doped Self-Supporting Gallium Nitride Substrate

A seed crystal substrate in which a GaN film having a thickness of 3 μmwas stacked on the oriented alumina substrate was produced as in (3a) ofExample 1. A Ge-doped GaN film was formed on this seed crystal substrateas in (3b) of Example 1 except that the retention time was 30 hours. Inthe resulting sample, Ge-doped gallium nitride crystals grew on theentire surface of the 50.8 mm (2 inches) seed crystal substrate, and thecrystal thickness was about 0.3 mm. No cracks were observed.

The oriented alumina substrate portion of the sample obtained in thisway was removed by grinding with grinding wheel to obtain a Ge-dopedgallium nitride single body. Then, the bottom surface (the surface onthe side that had been in contact with the oriented alumina substrate)of the Ge-doped gallium nitride crystals was ground by about 80 μm using#600 and #2000 grinding wheels. Thereafter, the plate surface (topsurface) was flattened by grinding until the thickness of the galliumnitride crystals became 60 μm, and then the top surface and the bottomsurface were smoothed by lapping using diamond abrasive grains to obtaina self-supporting gallium nitride substrate having a thickness of about60 μm. The average roughness Ra of the top surface and the bottomsurface of the self-supporting gallium nitride substrate afterprocessing was 0.2 nm.

Next, inverse pole figure mapping of the cross section of the galliumnitride crystals was performed using a method similar to Examples 4 to8, and it was found that the gallium nitride crystals had a larger graindiameter on the top surface side (the side opposite to the orientedalumina substrate) than the oriented alumina substrate side, and theshape of the gallium nitride crystals, being trapezoidal, triangular, orthe like on the cross-sectional image, was not completely columnar.Moreover, it was found that, as the film thickened, there were grainswhose grain diameter increased and that grew to the top surface, andgrains that did not grow to the top surface. Although the cause of sucha growth behavior is not clear, it is considered that, as shown in FIG.5, such a behavior is a result of growth that progressed in such amanner that fast-growing grains covered slow-growing grains. Therefore,among the gallium nitride grains constituting the gallium nitridecrystals, grains exposed on the top surface side connect to the bottomsurface without intervention of a grain boundary, but grains exposed onthe bottom surface side include grains which ceased to grow halfway.

The volume resistivity was measured by a method similar to (3) ofExample 1, and the volume resistivity was 1×10⁻² Ω·cm. Moreover, as aresult of measuring the cross-sectional average diameter of GaN singlecrystal grains at the top surface and the bottom surface of theself-supporting gallium nitride substrate using a method similar to (3)of Example 1, the cross-sectional average diameter at the top surfacewas about 81 μm, and the cross-sectional average diameter at the bottomsurface was about 61 μm. In this way, the cross-sectional averagediameter was larger at the top surface than the bottom surface, andD_(T)/D_(B), which is the ratio of the cross-sectional average diameterD_(T) at the substrate top surface to the cross-sectional averagediameter D_(B) of the substrate bottom surface, was about 1.3. Moreover,the aspect ratio of GaN single crystal grains calculated as the ratio ofthe thickness of GaN crystals to the cross-sectional average diameter atthe top surface was about 0.7.

(3) Production of Light Emitting Device Including Ge-DopedSelf-Supporting Gallium Nitride Substrate

A light emitting functional layer was produced on the self-supportinggallium nitride substrate as in (4a) of Example 1, and measurement ofthe cross-sectional average diameter of single crystal grains at theoutermost surface resulted in a cross-sectional average diameter ofabout 81 μm. Moreover, as a result of producing a vertical lightemitting device as in (4b) of Example 1, rectifying characteristics wereconfirmed by I-V measurement between the cathode electrode and the anodeelectrode, and application of electricity in the forward directionconfirmed emission of light having a wavelength of 450 nm. However, itwas found that the luminance was somewhat high but weaker than that inExample 8.

Example 10 (1) Production of c-Plane Oriented Alumina Sintered Body

A plate-shaped alumina powder (manufactured by Kinsei Matec Co., Ltd.,grade 10030), a fine alumina powder (manufactured by Taimei ChemicalsCo., Ltd., grade TM-DAR), and a magnesium oxide powder (manufactured byUbe Material Industries, Ltd., grade 500 A) were prepared as rawmaterials, and 5 parts by weight of the plate-shaped alumina powder, 95parts by weight of the fine alumina powder, and 0.025 parts by weight ofthe magnesium oxide powder were mixed to obtain an alumina raw material.Next, 8 parts by weight of a binder (polyvinyl butyral: product nameBM-2, manufactured by Sekisui Chemical Co., Ltd.), 4 parts by weight ofa plasticizer (DOP: di(2-ethylhexyl) phthalate, manufactured by KuroganeKasei Co., Ltd.), 2 parts by weight of a dispersing agent (RheodolSP-030, manufactured by Kao Corporation), and a dispersion medium (amixture of xylene and 1-butanol in a weight ratio of 1:1) were mixedwith 100 parts by weight of the alumina raw material. The amount of thedispersion medium was adjusted so that the slurry viscosity was 20000cP. The slurry prepared as above was formed into a sheet on a PET filmby a doctor blade method so as to have a thickness after drying of 100μm. The resulting tape was cut into circles having a diameter of 50.8 mm(2 inches), then 30 pieces were stacked and placed on an Al plate havinga thickness of 10 mm, and then vacuum packing was performed. This vacuumpack was subjected to isostatic pressing in hot water at 85° C. under apressure of 100 kgf/cm², and a disc-shaped green body was obtained.

The resulting green body was placed in a degreasing furnace anddegreased under conditions of 600° C. and 10 hours. The resultingdegreased body was fired with a hot pressing under conditions of 1800°C., 4 hours, and a surface pressure of 200 kgf/cm² in nitrogen using agraphite mold.

The sintered body obtained in this way was fixed to a ceramic surfaceplate and ground to #2000 using grinding wheel to flatten the platesurface. Next, the plate surface was smoothed by lapping using diamondabrasive grains to obtain an oriented alumina sintered body having adiameter of 50.8 mm (2 inches) and a thickness of 1 mm as an orientedalumina substrate. Flatness was improved by reducing the size ofabrasive grains from 3 μm to 0.5 μm in a stepwise manner. The averageroughness Ra after processing was 4 nm. Evaluation of the degree ofc-plane orientation and the average grain diameter at the plate surfaceby methods similar to Example 1 resulted in a degree of c-planeorientation of 99% and an average grain diameter of about 24 μm.

(2) Production of Ge-Doped Self-Supporting Gallium Nitride Substrate

A seed crystal substrate in which a GaN film having a thickness of 3 μmwas stacked on the oriented alumina substrate was produced as in (3a) ofExample 1. A Ge-doped GaN film was formed on this seed crystal substrateas in (3b) of Example 1 except that the retention time was 30 hours. Inthe resulting sample, Ge-doped gallium nitride crystals grew on theentire surface of the 50.8 mm (2 inches) seed crystal substrate, and thecrystal thickness was about 0.3 mm. No cracks were observed.

The oriented alumina substrate portion of the sample obtained in thisway was removed by grinding with grinding wheel to obtain a Ge-dopedgallium nitride single body. Then, the bottom surface (the surface onthe side that had been in contact with the oriented alumina substrate)of the Ge-doped gallium nitride crystals was ground by about 90 μm using#600 and #2000 grinding wheels. Thereafter, the plate surface (topsurface) was flattened by grinding until the thickness of the galliumnitride crystals became 40 μm, and then the top surface and the bottomsurface were smoothed by lapping using diamond abrasive grains to obtaina self-supporting gallium nitride substrate having a thickness of about40 μm. The average roughness Ra of the top surface and the bottomsurface of the self-supporting gallium nitride substrate afterprocessing was 0.2 nm.

Next, inverse pole figure mapping of the cross section of the galliumnitride crystals was performed using a method similar to Examples 4 to9, and it was found that the gallium nitride crystals had a larger graindiameter on the top surface side (the side opposite to the orientedalumina substrate) than the oriented alumina substrate side, and theshape of the gallium nitride crystals, being trapezoidal, triangular, orthe like on the cross-sectional image, was not completely columnar.Moreover, it was found that, as the film thickened, there were grainswhose grain diameter increased and that grew to the top surface, andgrains that did not grow to the top surface. Although the cause of sucha growth behavior is not clear, it is considered that, as shown in FIG.5, such a behavior is a result of growth that progressed in such amanner that fast-growing grains covered slow-growing grains. Therefore,among the gallium nitride grains constituting the gallium nitridecrystals, grains exposed on the top surface side connect to the bottomsurface without intervention of a grain boundary, but grains exposed onthe bottom surface side include grains which ceased to grow halfway.

The volume resistivity was measured by a method similar to (3) ofExample 1, and the volume resistivity was 1×10² Ω·cm. Moreover, as aresult of measuring the cross-sectional average diameter of GaN singlecrystal grains at the top surface and the bottom surface of theself-supporting gallium nitride substrate using a method similar to (3)of Example 1, the cross-sectional average diameter at the top surfacewas about 75 μm, and the cross-sectional average diameter at the bottomsurface was about 60 μm. In this way, the cross-sectional averagediameter was larger at the top surface than the bottom surface, andD_(T)/D_(B), which is the ratio of the cross-sectional average diameterD_(T) at the substrate top surface to the cross-sectional averagediameter D_(B) of the substrate bottom surface, was about 1.3. Moreover,the aspect ratio of GaN single crystal grains calculated as the ratio ofthe thickness of GaN crystals to the cross-sectional average diameter atthe top surface was about 0.5.

(3) Production of Light Emitting Device Including Ge-DopedSelf-Supporting Gallium Nitride Substrate

A light emitting functional layer was produced on the self-supportinggallium nitride substrate as in (4a) of Example 1, and measurement ofthe cross-sectional average diameter of single crystal grains at theoutermost surface resulted in a cross-sectional average diameter ofabout 75 μm. Moreover, as a result of producing a vertical lightemitting device as in (4b) of Example 1, rectifying characteristics wereconfirmed by I-V measurement between the cathode electrode and the anodeelectrode, and application of electricity in the forward directionconfirmed emission of light having a wavelength of 450 nm. However, itwas found that the luminance was somewhat high but weaker than that inExamples 8 and 9.

1. A self-supporting gallium nitride substrate composed of a platecomposed of a plurality of gallium nitride-based single crystal grains,wherein the plate has a single crystal structure in an approximatelynormal direction, wherein an aspect ratio T/D_(T), which is defined as aratio of a thickness T of the self-supporting gallium nitride substrateto a cross-sectional average diameter D_(T) at an outermost surface ofthe gallium nitride-based single crystal grains exposed at a top surfaceof the self-supporting gallium nitride substrate, is 0.7 or greater. 2.The self-supporting gallium nitride substrate according to claim 1,wherein a cross-sectional average diameter of the gallium nitride-basedsingle crystal grains at an outermost surface of the substrate is 0.3 μmor greater.
 3. The self-supporting gallium nitride substrate accordingto claim 2, wherein the cross-sectional average diameter is 3 μm orgreater.
 4. The self-supporting gallium nitride substrate according toclaim 2, wherein the cross-sectional average diameter is 20 μm orgreater.
 5. The self-supporting gallium nitride substrate according toclaim 1, having a thickness of 20 μm or greater.
 6. The self-supportinggallium nitride substrate according to claim 1, having a diameter of 100mm or greater.
 7. The self-supporting gallium nitride substrateaccording to claim 1, wherein the gallium nitride-based single crystalgrains have crystal orientation that is mostly aligned in theapproximately normal direction.
 8. The self-supporting gallium nitridesubstrate according to claim 1, wherein the gallium nitride-based singlecrystal grains are doped with an n-type dopant or a p-type dopant. 9.The self-supporting gallium nitride substrate according to claim 1,wherein the gallium nitride-based single crystal grains are free from adopant.
 10. The self-supporting gallium nitride substrate according toclaim 1, wherein the gallium nitride-based single crystal grains aremade of a mixed crystal.
 11. The self-supporting gallium nitridesubstrate according to claim 1, wherein the gallium nitride-based singlecrystal grains exposed at a top surface of the self-supporting galliumnitride substrate connect to a bottom surface of the self-supportinggallium nitride substrate without intervention of a grain boundary. 12.The self-supporting gallium nitride substrate according to claim 1,wherein a ratio D_(T)/D_(B), which is a ratio of a cross-sectionalaverage diameter D_(T) at an outermost surface of the galliumnitride-based single crystal grains exposed at a top surface of theself-supporting gallium nitride substrate to a cross-sectional averagediameter D_(B) at an outermost surface of the gallium nitride-basedsingle crystal grains exposed at a bottom surface of the self-supportinggallium nitride substrate, is greater than 1.0.
 13. The self-supportinggallium nitride substrate according to claim 1, wherein the aspect ratioT/D_(T) is 1.0 or greater.
 14. A light emitting device comprising: theself-supporting gallium nitride substrate according to claim 1; and alight emitting functional layer formed on the substrate, wherein thelight emitting functional layer has at least one layer composed of aplurality of semiconductor single crystal grains, wherein the at leastone layer has a single crystal structure in an approximately normaldirection.
 15. The light emitting device according to claim 14, whereina cross-sectional average diameter of the semiconductor single crystalgrains at an outermost surface of the light emitting functional layer is0.3 μm or greater.
 16. The light emitting device according to claim 15,wherein the cross-sectional average diameter is 3 μm or greater.
 17. Thelight emitting device according to claim 14, wherein the semiconductorsingle crystal grains have a structure in which grains are grown mostlyin conformity with crystal orientation of the self-supporting galliumnitride substrate.
 18. The light emitting device according to claim 14,wherein the light emitting functional layer is composed of a galliumnitride-based material.
 19. A method for manufacturing a self-supportinggallium nitride substrate, comprising the steps of: providing anoriented polycrystalline sintered body; forming a seed crystal layercomposed of gallium nitride on the oriented polycrystalline sinteredbody so that the seed crystal layer has crystal orientation mostly inconformity with crystal orientation of the oriented polycrystallinesintered body; forming a layer with a thickness of 20 μm or greatercomposed of gallium nitride-based crystals on the seed crystal layer sothat the layer has crystal orientation mostly in conformity with crystalorientation of the seed crystal layer; and removing the orientedpolycrystalline sintered body to obtain the self-supporting galliumnitride substrate, wherein an aspect ratio T/D_(T), which is defined asa ratio of a thickness T of the self-supporting gallium nitridesubstrate to a cross-sectional average diameter D_(T) at an outermostsurface of the gallium nitride-based single crystal grains exposed at atop surface of the self-supporting gallium nitride substrate, is 0.7 orgreater.
 20. The method according to claim 19, wherein the orientedpolycrystalline sintered body is an oriented polycrystalline aluminasintered body.
 21. The method according to claim 19, wherein an averagegrain diameter of grains constituting the oriented polycrystallinesintered body on a plate surface is 0.3 to 1000 μm.
 22. The methodaccording to claim 19, wherein formation of the layer composed of thegallium nitride-based crystals is performed by Na flux method.
 23. Themethod according to claim 19, wherein the oriented polycrystallinesintered body is transparent or translucent.
 24. A method formanufacturing a light emitting device, comprising the steps of:providing the self-supporting gallium nitride substrate of claim 1; andforming on the self-supporting gallium nitride substrate at least onelayer composed of a plurality of semiconductor single crystal grains,wherein the at least one layer has a single crystal structure in anapproximately normal direction, so that the at least one layer hascrystal orientation mostly in conformity with crystal orientation of theself-supporting gallium nitride substrate, thereby providing a lightemitting functional layer.
 25. The method according to claim 24, whereinthe light emitting functional layer is composed of a galliumnitride-based material.